Employing pretreatment and fluorination of fluoropolymer resin to reduce discoloration

ABSTRACT

Process for reducing thermally induced discoloration of fluoropolymer resin produced by polymerizing fluoromonomer in an aqueous dispersion medium to form aqueous fluoropolymer dispersion and isolating said fluoropolymer from the aqueous medium by separating fluoropolymer resin in wet form from the aqueous medium and drying to produce fluoropolymer resin in dry form. The process comprises:
         pretreating the aqueous fluoropolymer dispersion and/or the fluoropolymer resin in wet or dry form; and   exposing the fluoropolymer resin in dry form to fluorine.

FIELD OF THE INVENTION

This invention relates to a process for reducing thermally induceddiscoloration of fluoropolymer resin.

BACKGROUND OF THE INVENTION

A typical process for the aqueous dispersion polymerization offluorinated monomer to produce fluoropolymer includes feedingfluorinated monomer to a heated reactor containing an aqueous medium andadding a free-radical initiator to commence polymerization. Afluorosurfactant is typically employed to stabilize the fluoropolymerparticles formed. After several hours, the feeds are stopped, thereactor is vented and purged with nitrogen, and the raw dispersion inthe vessel is transferred to a cooling vessel.

The fluoropolymer formed can be isolated from the dispersion to obtainfluoropolymer resin. For example, polytetrafluoroethylene (PTFE) resinreferred to as PTFE fine powder is produced by isolating PTFE resin fromPTFE dispersion by coagulating the dispersion to separate PTFE from theaqueous medium and then drying. Dispersions of melt-processiblefluoropolymers such as copolymers of tetrafluoroethylene andhexafluoropropylene (FEP) and tetrafluoroethylene and perfluoro (alkylvinyl ethers) (PFA) useful as molding resins can be similarly coagulatedand the coagulated polymer is dried and then used directly inmelt-processing operations or melt-processed into a convenient form suchas chip or pellet for use in subsequent melt-processing operations.

Because of environmental concerns relating to fluorosurfactants, thereis interest in using hydrocarbon surfactants in the aqueouspolymerization medium in place of a portion of or all of thefluorosurfactant. However, when fluoropolymer dispersion is formed whichcontains hydrocarbon surfactant and is subsequently isolated to obtainfluoropolymer resin, the fluoropolymer resin is prone to thermallyinduced discoloration. By thermally induced discoloration is meant thatundesirable color forms or increases in the fluoropolymer resin uponheating. It is usually desirable for fluoropolymer resin to be clear orwhite in color and, in resin prone to thermally induced discoloration, agray or brown color, sometimes quite dark forms upon heating. Forexample, if PTFE fine power produced from dispersion containing thehydrocarbon surfactant sodium dodecyl sulfate (SDS) is converted intopaste-extruded shapes or films and subsequently sintered, an undesirablegray or brown color will typically arise. Color formation upon sinteringin PTFE produced from dispersion containing the hydrocarbon surfactantSDS has been described in Example VI of U.S. Pat. No. 3,391,099 toPunderson. Similarly, when melt processible fluoropolymers such as FEPor PFA are produced from dispersions containing hydrocarbon surfactantsuch as SDS, undesirable color typically occurs when the fluoropolymeris first melt-processed, for example, when melt processed into aconvenient form for subsequent use such as chip or pellet.

SUMMARY OF THE INVENTION

Process for reducing thermally induced discoloration of fluoropolymerresin produced by polymerizing fluoromonomer in an aqueous dispersionmedium to form aqueous fluoropolymer dispersion and isolating saidfluoropolymer from the aqueous medium by separating fluoropolymer resinin wet form from the aqueous medium and drying to produce fluoropolymerresin in dry form. The process comprises:

pretreating the aqueous fluoropolymer dispersion and/or thefluoropolymer resin in wet or dry form; and

exposing the fluoropolymer resin in dry form to fluorine.

Preferably, the process reduces the thermally induced discoloration byat least about 10% as measured by % change in L* on the CIELAB colorscale.

The process of the invention is useful for fluoropolymer resin whichexhibits thermally induced discoloration which ranges from mild tosevere. The process of the invention may be employed for fluoropolymerresin which exhibits thermally induced discoloration prior to treatmentwhich is significantly greater than equivalent fluoropolymer resin ofcommercial quality manufactured using ammonium perfluorooctanoatefluorosurfactant. The process of the invention is advantageouslyemployed when the fluoropolymer resin has an initial thermally induceddiscoloration value (L*_(i)) at least about 4 L units on the CIELABcolor scale below the L* value of equivalent fluoropolymer resin ofcommercial quality manufactured using ammonium perfluorooctanoatefluorosurfactant.

The invention is particularly useful for fluoropolymer resin obtainedfrom aqueous fluoropolymer dispersion made by polymerizing fluoromonomercontaining hydrocarbon surfactant which causes thermally induceddiscoloration, preferably aqueous fluoropolymer dispersion polymerizedin the presence of hydrocarbon surfactant.

DETAILED DESCRIPTION OF THE INVENTION

Fluoromonomer/Fluoropolymer

Fluoropolymer resins are produced by polymerizing fluoromonomer in anaqueous medium to form aqueous fluoropolymer dispersion. Thefluoropolymer is made from at least one fluorinated monomer(fluoromonomer), i.e., wherein at least one of the monomers containsfluorine, preferably an olefinic monomer with at least one fluorine or afluoroalkyl group attached to a doubly-bonded carbon. The fluorinatedmonomer and the fluoropolymer obtained therefrom each preferably containat least 35 wt % F, preferably at least 50 wt % F and the fluorinatedmonomer is preferably independently selected from the group consistingof tetrafluoroethylene (TFE), hexafluoropropylene (HFP),chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluoroalkyl ethylene, fluorovinyl ethers,vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD),perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD), perfluoro(allylvinyl ether) and perfluoro(butenyl vinyl ether) and mixtures thereof. Apreferred perfluoroalkyl ethylene monomer is perfluorobutyl ethylene(PFBE). Preferred fluorovinyl ethers include perfluoro(alkyl vinylether) monomers (PAVE) such as perfluoro(propyl vinyl ether) (PPVE),perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether)(PMVE). Non-fluorinated olefinic comonomers such as ethylene andpropylene can be copolymerized with fluorinated monomers.

Fluorovinyl ethers also include those useful for introducingfunctionality into fluoropolymers. These includeCF₂═CF—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₂F, wherein R_(f) and R′_(f)are independently selected from F, Cl or a perfluorinated alkyl grouphaving 1 to 10 carbon atoms, a=0, 1 or 2. Polymers of this type aredisclosed in U.S. Pat. No. 3,282,875 (CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F,perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride)), and in U.S.Pat. Nos. 4,358,545 and 4,940,525 (CF₂═CF—O—CF₂CF₂SO₂F). Another exampleis CF₂═CF—O—CF₂—CF(CF₃)—O—CF₂CF₂CO₂CH₃, methyl ester ofperfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid), disclosed in U.S.Pat. No. 4,552,631. Similar fluorovinyl ethers with functionality ofnitrile, cyanate, carbamate, and phosphonic acid are disclosed in U.S.Pat. Nos. 5,637,748; 6,300,445; and 6,177,196.

A preferred class of fluoropolymers useful for reducing thermallyinduced discoloration is perfluoropolymers in which the monovalentsubstituents on the carbon atoms forming the chain or backbone of thepolymer are all fluorine atoms, with the possible exception ofcomonomer, end groups, or pendant group structure. Preferably thecomonomer, end group, or pendant group structure will impart no morethan 2 wt % C—H moiety, more preferably no greater than 1 wt % C—Hmoiety, with respect to the total weight of the perfluoropolymer.Preferably, the hydrogen content, if any, of the perfluoropolymer is nogreater than 0.2 wt %, based on the total weight of theperfluoropolymer.

The invention is useful for reducing thermally induced discoloration offluoropolymers of polytetrafluoroethylene (PTFE) including modifiedPTFE. Polytetrafluoroethylene (PTFE) refers to (a) the polymerizedtetrafluoroethylene by itself without any significant comonomer present,i.e. homopolymer and (b) modified PTFE, which is a copolymer of TFEhaving such small concentrations of comonomer that the melting point ofthe resultant polymer is not substantially reduced below that of PTFE.The modified PTFE contains a small amount of comonomer modifier whichreduces crystallinity to improve film forming capability during baking(fusing). Examples of such monomers include perfluoroolefin, notablyhexafluoropropylene (HFP) or perfluoro(alkyl vinyl ether) (PAVE), wherethe alkyl group contains 1 to 5 carbon atoms, with perfluoro(ethyl vinylether) (PEVE) and perfluoro(propyl vinyl ether) (PPVE) being preferred,chlorotrifluoroethylene (CTFE), perfluorobutyl ethylene (PFBE), or othermonomer that introduces bulky side groups into the polymer molecule. Theconcentration of such comonomer is preferably less than 1 wt %, morepreferably less than 0.5 wt %, based on the total weight of the TFE andcomonomer present in the PTFE. A minimum amount of at least about 0.05wt % is preferably used to have significant effect. PTFE (and modifiedPTFE) typically have a melt creep viscosity of at least about 1×10⁶ Pa·sand preferably at least 1×10⁸ Pa·s and, with such high melt viscosity,the polymer does not flow in the molten state and therefore is not amelt-processible polymer. The measurement of melt creep viscosity isdisclosed in col. 4 of U.S. Pat. No. 7,763,680. The high melt viscosityof PTFE arises from is extremely high molecular weight (Mn), e.g. atleast 10⁶. PTFE can also be characterized by its high meltingtemperature, of at least 330° C., upon first heating. The non-meltflowability of the PTFE, arising from its extremely high melt viscosity,results in a no melt flow condition when melt flow rate (MFR) ismeasured in accordance with ASTM D 1238 at 372° C. and using a 5 kgweight, i.e., MFR is 0. The high molecular weight of PTFE ischaracterized by measuring its standard specific gravity (SSG). The SSGmeasurement procedure (ASTM D 4894, also described in U.S. Pat. No.4,036,802) includes sintering of the SSG sample free standing (withoutcontainment) above its melting temperature without change in dimensionof the SSG sample. The SSG sample does not flow during the sintering.

The process of the present invention is also useful in reducingthermally induced discoloration of low molecular weight PTFE, which iscommonly known as PTFE micropowder, so as to distinguish from the PTFEdescribed above. The molecular weight of PTFE micropowder is lowrelative to PTFE, i.e. the molecular weight (Mn) is generally in therange of 10⁴ to 10⁵. The result of this lower molecular weight of PTFEmicropowder is that it has fluidity in the molten state, in contrast toPTFE which is not melt flowable. PTFE micropowder has melt flowability,which can be characterized by a melt flow rate (MFR) of at least 0.01g/10 min, preferably at least 0.1 g/10 min and more preferably at least5 g/10 min, and still more preferably at least 10 g/10 min., as measuredin accordance with ASTM D 1238, at 372° C. using a 5 kg weight on themolten polymer.

The invention is especially useful for reducing thermally induceddiscoloration of melt-processible fluoropolymers that are alsomelt-fabricable. Melt-processible means that the fluoropolymer can beprocessed in the molten state, i.e., fabricated from the melt usingconventional processing equipment such as extruders and injectionmolding machines, into shaped articles such as films, fibers, and tubes.Melt-fabricable means that the resultant fabricated articles exhibitsufficient strength and toughness to be useful for their intendedpurpose. This sufficient strength may be characterized by thefluoropolymer by itself exhibiting an MIT Flex Life of at least 1000cycles, preferably at least 2000 cycles, measured as disclosed in U.S.Pat. No. 5,703,185. The strength of the fluoropolymer is indicated by itnot being brittle.

Examples of such melt-processible fluoropolymers include homopolymerssuch as polychlorotrifluoroethylene and polyvinylidene fluoride (PVDF)or copolymers of tetrafluoroethylene (TFE) and at least one fluorinatedcopolymerizable monomer (comonomer) present in the polymer usually insufficient amount to reduce the melting point of the copolymersubstantially below that of PTFE, e.g., to a melting temperature nogreater than 315° C.

A melt-processible TFE copolymer typically incorporates an amount ofcomonomer into the copolymer in order to provide a copolymer which has amelt flow rate (MFR) of 0.1 to 200 g/10 min as measured according toASTM D-1238 using a 5 kg weight on the molten polymer and the melttemperature which is standard for the specific copolymer. MFR willpreferably range from 1 to 100 g/10 min, most preferably about 1 toabout 50 g/10 min. Additional melt-processible fluoropolymers are thecopolymers of ethylene (E) or propylene (P) with TFE or CTFE, notablyETFE and ECTFE.

A preferred melt-processible copolymer for use in the practice of thepresent invention comprises at least 40-99 mol % tetrafluoroethyleneunits and 1-60 mol % of at least one other monomer. Additionalmelt-processible copolymers are those containing 60-99 mol % PTFE unitsand 1-40 mol % of at least one other monomer. Preferred comonomers withTFE to form perfluoropolymers are perfluoromonomers, preferablyperfluoroolefin having 3 to 8 carbon atoms, such as hexafluoropropylene(HFP), and/or perfluoro(alkyl vinyl ether) (PAVE) in which the linear orbranched alkyl group contains 1 to 5 carbon atoms. Preferred PAVEmonomers are those in which the alkyl group contains 1, 2, 3 or 4 carbonatoms, and the copolymer can be made using several PAVE monomers.Preferred TFE copolymers include FEP (TFE/HFP copolymer), PFA (TFE/PAVEcopolymer), TFE/HFP/PAVE wherein PAVE is PEVE and/or PPVE, MFA(TFE/PMVE/PAVE wherein the alkyl group of PAVE has at least two carbonatoms) and THV (TFE/HFP/VF₂).

All these melt-processible fluoropolymers can be characterized by MFR asrecited above for the melt-processible TFE copolymers, i.e. by theprocedure of ASTM 1238 using standard conditions for the particularpolymer, including a 5 kg weight on the molten polymer in theplastometer for the MFR determination of PFA and FEP

Further useful polymers are film forming polymers of polyvinylidenefluoride (PVDF) and copolymers of vinylidene fluoride as well aspolyvinyl fluoride (PVF) and copolymers of vinyl fluoride.

The invention is also useful when reducing thermally induceddiscoloration of fluorocarbon elastomers (fluoroelastomers). Theseelastomers typically have a glass transition temperature below 25° C.and exhibit little or no crystallinity at room temperature and little orno melting temperature. Fluoroelastomer made by the process of thisinvention typically are copolymers containing 25 to 75 wt %, based ontotal weight of the fluoroelastomer, of copolymerized units of a firstfluorinated monomer which may be vinylidene fluoride (VF₂) ortetrafluoroethylene (TFE). The remaining units in the fluoroelastomersare comprised of one or more additional copolymerized monomers,different from the first monomer, selected from the group consisting offluorinated monomers, hydrocarbon olefins and mixtures thereof.Fluoroelastomers may also, optionally, comprise units of one or morecure site monomers. When present, copolymerized cure site monomers aretypically at a level of 0.05 to 7 wt %, based on total weight offluorocarbon elastomer. Examples of suitable cure site monomers include:i) bromine-, iodine-, or chlorine-containing fluorinated olefins orfluorinated vinyl ethers; ii) nitrile group-containing fluorinatedolefins or fluorinated vinyl ethers; iii) perfluoro(2-phenoxypropylvinyl ether); and iv) non-conjugated dienes.

Preferred TFE based fluoroelastomer copolymers include TFE/PMVE,TFE/PMVE/E, TFE/P and TFE/P/VF₂. Preferred VF₂ based fluorocarbonelastomer copolymers include VF₂/HFP, VF₂/HFP/TFE, and VF₂/PMVE/TFE. Anyof these elastomer copolymers may further comprise units of cure sitemonomer.

Hydrocarbon Surfactants

In one embodiment of the present invention, the aqueous fluoropolymerdispersion medium used to form fluoropolymer resin contains hydrocarbonsurfactant which causes thermally induced discoloration in the resinwhen the fluoropolymer resin is isolated and heated. The hydrocarbonsurfactant is a compound that has hydrophobic and hydrophilic moieties,which enables it to disperse and stabilize hydrophobic fluoropolymerparticles in an aqueous medium. The hydrocarbon surfactant is preferablyan anionic surfactant. An anionic surfactant has a negatively chargedhydrophilic portion such as a carboxylate, sulfonate, or sulfate saltand a long chain hydrocarbon portion, such as alkyl as the hydrophobicportion. Hydrocarbon surfactants often serve to stabilize polymerparticles by coating the particles with the hydrophobic portion of thesurfactant oriented towards the particle and the hydrophilic portion ofthe surfactant in the water phase. The anionic surfactant adds to thisstabilization because it is charged and provides repulsion of theelectrical charges between polymer particles. Surfactants typicallyreduce surface tension of the aqueous medium containing the surfactantsignificantly.

One example anionic hydrocarbon surfactant is the highly branched C10tertiary carboxylic acid supplied as Versatic® 10 by ResolutionPerformance Products.

Versatic® 10

-   -   Neodecanoic acid (n+m=7)

Another useful anionic hydrocarbon surfactant is the sodium linear alkylpolyether sulfonates supplied as the Avanel® S series by BASF. Theethylene oxide chain provides nonionic characteristics to the surfactantand the sulfonate groups provide certain anionic characteristics.

Avanel®

-   -   S-70 (n=7, m=11-14)    -   S-74 (n=3, m=8)

Another group of hydrocarbon surfactants are those anionic surfactantsrepresented by the formula R-L-M wherein R is preferably a straightchain alkyl group containing from 6 to 17 carbon atoms, L is selectedfrom the group consisting of —ArSO₃ ⁻, —SO₃ ⁻, —SO₄ ⁻, —PO₃ ⁻, —PO₄ ⁻and —COO⁻, and M is a univalent cation, preferably H⁺, Na⁺, K⁺ and NH₄⁺. —ArSO₃ ⁻ is aryl sulfonate. Preferred of these surfactants are thoserepresented by the formula CH₃—(CH₂)_(n)-L-M, wherein n is an integer of6 to 17 and L is selected from —SO₄M, —PO₃M, —PO₄M, or —COOM and L and Mhave the same meaning as above. Especially preferred are R-L-Msurfactants wherein the R group is an alkyl group having 12 to 16 carbonatoms and wherein L is sulfate, and mixtures thereof. Especiallypreferred of the R-L-M surfactants is sodium dodecyl sulfate (SDS). Forcommercial use, SDS (sometimes referred to as sodium lauryl sulfate orSLS), is typically obtained from coconut oil or palm kernel oilfeedstocks, and contains predominately sodium dodecyl sulfate but maycontain minor quantities of other R-L-M surfactants with differing Rgroups. “SDS” as used in this application means sodium dodecyl sulfateor surfactant mixtures which are predominantly sodium docecyl sulphatecontaining minor quantities of other R-L-M surfactants with differing Rgroups.

Another example of anionic hydrocarbon surfactant useful in the presentinvention is the sulfosuccinate surfactant Lankropol® K8300 availablefrom Akzo Nobel Surface Chemistry LLC. The surfactant is reported to bethe following:

Butanedioic acid, sulfo-,4-(1-methyl-2-((1-oxo-9-octadecenyl)amino)ethyl) ester, disodium salt;CAS No.: 67815-88-7

Additional sulfosuccinate hydrocarbon surfactants useful in the presentinvention are diisodecyl sulfosuccinate, Na salt, available asEmulsogen® SB10 from Clariant, and diisotridecyl sulfosuccinate, Nasalt, available as Polirol® TR/LNA from Cesapinia Chemicals.

Another preferred class of hydrocarbon surfactants is nonionicsurfactants. A nonionic surfactant does not contain a charged group buthas a hydrophobic portion that is typically a long chain hydrocarbon.The hydrophilic portion of the nonionic surfactant typically containswater soluble functionality such as a chain of ethylene ether derivedfrom polymerization with ethylene oxide. In the stabilization context,surfactants stabilize polymer particles by coating the particles withthe hydrophobic portion of the surfactant oriented towards the particleand the hydrophilic portion of the surfactant in the water phase.

Nonionic hydrocarbon surfactants include polyoxyethylene alkyl ethers,polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl esters,sorbitan alkyl esters, polyoxyethylene sorbitan alkyl esters, glycerolesters, their derivatives and the like. More specifically examples ofpolyoxyethylene alkyl ethers are polyoxyethylene lauryl ether,polyoxyethylene cetyl ether, polyoxyethylene stearyl ether,polyoxyethylene oleyl ether, polyoxyethylene behenyl ether and the like;examples of polyoxyethylene alkyl phenyl ethers are polyoxyethylenenonyl phenyl ether, polyoxyethylene octyl phenyl ether and the like;examples of polyoxyethylene alkyl esters are polyethylene glycolmonolaurylate, polyethylene glycol monooleate, polyethylene glycolmonostearate and the like; examples of sorbitan alkyl esters arepolyoxyethylene sorbitan monolaurylate, polyoxyethylene sorbitanmonopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylenesorbitan monooleate and the like; examples of polyoxyethylene sorbitanalkyl esters are polyoxyethylene sorbitan monolaurylate, polyoxyethylenesorbitan monopalmitate, polyoxyethylene sorbitan monostearate and thelike; and examples of glycerol esters are glycerol monomyristate,glycerol monostearate, glycerol monooleate and the like. Also examplesof their derivatives are polyoxyethylene alkyl amine, polyoxyethylenealkyl phenyl-formaldehyde condensate, polyoxyethylene alkyl etherphosphate and the like. Particularly preferable are polyoxyethylenealkyl ethers and polyoxyethylene alkyl esters. Examples of such ethersand esters are those that have an HLB value of 10 to 18. Moreparticularly there are polyoxyethylene lauryl ether (EO: 5 to 20. EOstands for an ethylene oxide unit.), polyethylene glycol monostearate(EO: 10 to 55) and polyethylene glycol monooleate (EO: 6 to 10).

Suitable nonionic hydrocarbon surfactants include octyl phenolethoxylates such as the Triton® X series supplied by Dow ChemicalCompany:

Triton®

-   -   X15 (n˜1.5)    -   X45 (n˜4.5)    -   X100 (n˜10)

Preferred nonionic hydrocarbon surfactants are branched alcoholethoxylates such as the Tergitol® 15-S series supplied by Dow ChemicalCompany and branched secondary alcohol ethoxylates such as the Tergitol®TMN series also supplied by Dow Chemical Company:

-   Tergitol®-   TMN-6 (n˜8)-   TMN-10 (n˜11)-   TMN-100 (n˜10)

Ethyleneoxide/propylene oxide copolymers such as the Tergitol® L seriessurfactant supplied by Dow Chemical Company are also useful as nonionicsurfactants in this invention.

Yet another useful group of suitable nonionic hydrocarbon surfactantsare difunctional block copolymers supplied as Pluronic® R series fromBASF, such as:

Pluronic® R

-   -   31R1 (m˜26, n˜8)    -   17R2 (m˜14, n˜9)    -   10R5 (m˜8, n˜22)    -   25R4 (m˜22, n˜23)

Another group of suitable nonionic hydrocarbon surfactants are tridecylalcohol alkoxylates supplied as Iconol® TDA series from BASFCorporation.

Iconol®

-   -   TDA-6 (n=6)    -   TDA-9 (n=9)    -   TDA-10 (n=10)

In a preferred embodiment, all of the monovalent substituents on thecarbon atoms of the hydrocarbon surfactants are hydrogen. Thehydrocarbon is surfactant is preferably essentially free of halogensubstituents, such as fluorine or chlorine. Accordingly, the monovalentsubstituents, as elements from the Periodic Table, on the carbon atomsof the surfactant are at least 75%, preferably at least 85%, and morepreferably at least 95% hydrogen. Most preferably, 100% of themonovalent substituents as elements of the Periodic Table, on the carbonatoms are hydrogen. However, in one embodiment, a number of carbon atomscan contain halogen atoms in a minor amount.

Examples of hydrocarbon-containing surfactants useful in the presentinvention in which only a minor number of monovalent substituents oncarbon atoms are fluorine instead of hydrogen are the PolyFox®surfactants available from Omnova Solutions, Inc., described below

Polymerization Process

For the practice of the present invention, fluoropolymer resin isproduced by polymerizing fluoromonomer. Polymerization may be suitablycarried out in a pressurized polymerization reactor which producesaqueous fluoropolymer dispersion. A batch or continuous process may beused although batch processes are more common for commercial production.The reactor is preferably equipped with a stirrer for the aqueous mediumand a jacket surrounding the reactor so that the reaction temperaturemay be conveniently controlled by circulation of a controlledtemperature heat exchange medium. The aqueous medium is preferablydeionized and deaerated water. The temperature of the reactor and thusof the aqueous medium will preferably be from about 25 to about 120° C.

To carry out polymerization, the reactor is typically pressured up withfluoromonomer to increase the reactor internal pressure to operatingpressure which is generally in the range of about 30 to about 1000 psig(0.3 to 7.0 MPa). An aqueous solution of free-radical polymerizationinitiator can then be pumped into the reactor in sufficient amount tocause kicking off of the polymerization reaction, i.e. commencement ofthe polymerization reaction. The polymerization initiator employed ispreferably a water-soluble free-radical polymerization initiator. Forpolymerization of TFE to PTFE, preferred initiator is organic peracidsuch as disuccinic acid peroxide (DSP), which requires a large amount tocause kickoff, e.g. at least about 200 ppm, together with a highlyactive initiator, such as inorganic persulfate salt such as ammoniumpersulfate in a smaller amount. For TFE copolymers such as FEP and PFA,inorganic persulfate salt such as ammonium persulfate is generally used.The initiator added to cause kickoff can be supplemented by pumpingadditional initiator solution into the reactor as the polymerizationreaction proceeds.

For the production of modified PTFE and TFE copolymers, relativelyinactive fluoromonomer such as hexafluoropropylene (HFP) can already bepresent in the reactor prior to pressuring up with the more active TFEfluoromonomer. After kickoff, TFE is typically fed into the reactor tomaintain the internal pressure of the reactor at the operating pressure.Additional comonomer such as HFP or perfluoro (alkyl vinyl ether) can bepumped into the reactor if desired. The aqueous medium is typicallystirred to obtain a desired polymerization reaction rate and uniformincorporation of comonomer, if present. Chain transfer agents can beintroduced into the reactor when molecular weight control is desired.

In one embodiment of the present invention, the aqueous fluoropolymerdispersion is polymerized in the presence of hydrocarbon surfactant.Hydrocarbon surfactant is preferably present in the fluoropolymerdispersion because the aqueous fluoropolymer dispersion is polymerizedin the presence of hydrocarbon surfactant, i.e., hydrocarbon surfactantis used as a stabilizing surfactant during polymerization. If desiredfluorosurfactant such a fluoroalkane carboxylic acid or salt orfluoroether carboxylic acid or salt may be employed as stabilizingsurfactant together with hydrocarbon surfactant and therefore may alsopresent in the aqueous fluoropolymer dispersion produced. Preferably forthe practice of the present invention, the fluoropolymer dispersion ispreferably free of halogen-containing surfactant such asfluorosurfactant, i.e., contains less than about 300 ppm, and morepreferably less than about 100 ppm, and most preferably less than 50ppm, or halogen-containing surfactant.

In a polymerization process employing hydrocarbon surfactant as thestabilizing surfactant, addition of the stabilizing surfactant ispreferably delayed until after the kickoff has occurred. The amount ofthe delay will depend on the surfactant being used and the fluoromonomerbeing polymerized. In addition, it is preferably for the hydrocarbonsurfactant to be fed into the reactor as the polymerization proceeds,i.e., metered. The amount of hydrocarbon surfactant present in theaqueous fluoropolymer dispersion produced is preferably 10 ppm to about50,000 ppm, more preferably about 50 ppm to about 10,000 ppm, mostpreferably about 100 ppm to about 5000 ppm, based on fluoropolymersolids.

If desired, the hydrocarbon surfactant can be passivated prior to,during or after addition to the polymerization reactor. Passivatingmeans to reduce the telogenic behavior of the hydrocarbon-containingsurfactant. Passivation may be carried out by reacting said thehydrocarbon-containing surfactant with an oxidizing agent, preferablyhydrogen peroxide or polymerization initiator. Preferably, thepassivating of the hydrocarbon-containing surfactant is carried out inthe presence of a passivation adjuvant, preferably metal in the form ofmetal ion, most preferably, ferrous ion or cuprous ion.

After completion of the polymerization when the desired amount ofdispersed fluoropolymer or solids content has been achieved (typicallyseveral hours in a batch process), the feeds are stopped, the reactor isvented, and the raw dispersion of fluoropolymer particles in the reactoris transferred to a cooling or holding vessel.

The solids content of the aqueous fluoropolymer dispersion aspolymerized produced can range from about 10% by weight to up to about65 wt % by weight but typically is about 20% by weight to 45% by weight.Particle size (Dv(50)) of the fluoropolymer particles in the aqueousfluoropolymer dispersion can range from 10 nm to 400 nm, preferablyDv(50) about 100 to about 400 nm.

Isolation of the fluoropolymer includes separation of wet fluoropolymerresin from the aqueous fluoropolymer dispersion. Separation of the wetfluoropolymer resin from the aqueous fluoropolymer dispersion can beaccomplished by a variety of techniques including but not limited togelation, coagulation, freezing and thawing, and solvent aidedpelletization (SAP). When separation of wet fluoropolymer resin iscarried out by coagulation, the as polymerized dispersion may first bediluted from its as polymerized concentration. Stirring is then suitablyemployed to impart sufficient shear to the dispersion to causecoagulation and thereby produce undispersed fluoropolymer. Salts such asammonium carbonate can be added to the dispersion to assist withcoagulation if desired. Filtering can be used to remove at least aportion of the aqueous medium from the wet fluoropolymer resin.Separation can be performed by solvent aided pelletization as describedin U.S. Pat. No. 4,675,380 which produces granulated particles offluoropolymer.

Isolating the fluoropolymer typically includes drying to remove aqueousmedium which is retained in the fluoropolymer resin. After wetfluoropolymer resin is separated from the dispersion, fluoropolymerresin in wet form can include significant quantities of the aqueousmedium, for example, up to 60% by weight. Drying removes essentially allof the aqueous medium to produce fluoropolymer resin in dry form. Thewet fluoropolymer resin may be rinsed if desired and may be pressed toreduce aqueous medium content to reduce the energy and time required fordrying.

For melt processible fluoropolymers, wet fluoropolymer resin is driedand used directly in melt-processing operations or processed into aconvenient form such as chip or pellet for use in subsequentmelt-processing operations. Certain grades of PTFE dispersion are madefor the production of fine powder. For this use, the dispersion iscoagulated, the aqueous medium is removed and the PTFE is dried toproduce fine powder. For fine powder, conditions are suitably employedduring isolation which do not adversely affect the properties of thePTFE for end use processing. The shear in the dispersion during stirringis appropriately controlled and temperatures less than 200° C., wellbelow the sintering temperature of PTFE, are employed during drying.

Reduction of Thermally Induced Discoloration

To reduce thermally induced discoloration in accordance with the presentinvention, the aqueous fluoropolymer dispersion and/or the fluoropolymerresin, which may be in wet or dry form, is pretreated and thefluoropolymer resin in dry form is exposed to fluorine. As used in thisapplication, the term “and/or”, when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.Thus for the present invention, either of aqueous fluoropolymerdispersion, the fluoropolymer resin, or both the aqueous fluoropolymerdispersion and the fluoropolymer resin can be pretreated. Preferably,the process of the invention reduces the thermally induced discolorationby at least about 10% as measured by % change in L* on the CIELAB colorscale. As discussed in detail in the Test Methods which follow, the %change in L* of fluoropolymer resin samples is determined using theCIELAB color scale specified by International Commission on Illumination(CIE). More preferably, the process reduces the thermally induceddiscoloration by at least about 20% as measured by % change in L*, stillmore preferably at least about 30%, and most preferably at least about50%.

In accordance with the invention, the aqueous fluoropolymer dispersionand/or fluoropolymer resin are pretreated, preferably by exposing theaqueous fluoropolymer dispersion and/or the fluoropolymer resin to anoxidizing agent. In the practice of the present invention, thepretreatment may or may not result in reduction of thermally induceddiscoloration as measured by % change in L* if employed alone withoutsubsequently exposing the fluoropolymer resin in dry form to fluorine.Moreover, it is possible that the thermally induced discoloration of thefluoropolymer resin may be increased, i.e., the discoloration worsens,by the pretreatment alone without subsequently exposing thefluoropolymer resin in dry form to fluorine. However, the additiveeffect of the pretreatment in combination with exposing thefluoropolymer resin in dry form to fluorine in accordance with theinvention can provide an improvement over the reduction of thermallyinduced discoloration provided only by exposing the fluoropolymer resinin dry form to fluorine. The reduction of thermally induceddiscoloration measured by % change in L* on the CIELAB color scaleprovided by pretreating in combination with exposing the fluoropolymerresin in dry form to fluorine is preferably at least about 10% greaterthan the % change in L* on the CIELAB color scale provided by onlyexposing the fluoropolymer resin in dry form to same amount of fluorine,more preferably at least about 20% greater, still more preferably atleast about 30% greater, most preferably at least about 50% greater.Preferably, the pretreatment reduces the quantity of fluorine requiredto reduce thermally induced discoloration to a desired level.Preferably, for a given level of reduction of thermally induceddiscoloration, pretreating in combination with exposing thefluoropolymer resin in dry form to fluorine reduces the requiredquantity of fluorine by about 10% compared to only exposing thefluoropolymer resin in dry form to fluorine. More preferably, thepretreatment reduces the required quantity of fluorine by about 25%.Most preferably, the pretreatment reduces the required quantity offluorine by about 50%.

Pretreatment of the aqueous fluoropolymer dispersion can be accomplishedby a variety of techniques. One preferred pretreatment comprisesexposing the aqueous fluoropolymer dispersion to ultraviolet light inthe presence of an oxygen source. For the practice of this pretreatment,the aqueous fluoropolymer dispersion is preferably first diluted withwater to a concentration less than the concentration of the aspolymerized aqueous fluoropolymer dispersion because, depending upon theequipment used, exposure of ultraviolet light can be more effective forreducing discoloration for dilute dispersions. Preferred concentrationsare about 2 weight percent to about 30 weight percent, more preferablyabout 2 weight percent to about 20 weight percent.

Ultraviolet light has a wavelength range or about 10 nm to about 400 nmand has been described to have bands including: UVA (315 nm to 400 nm),UVB (280 nm to 315 nm), and UVC (100 nm to 280 nm). Preferably, theultraviolet light employed has a wavelength in the UVC band.

Any of various types of ultraviolet lamps can be used as the source ofultraviolet light. For example, submersible UV clarifier/sterilizerunits sold for the purposes of controlling algae and bacterial growth inponds are commercially available and may be used for the practice ofthis pretreatment. These units include a low-pressure mercury-vapor UVClamp within a housing for the circulation of water. The lamp isprotected by a quartz tube so that water can be circulated within thehousing for exposure to ultraviolet light. Submersible UVclarifier/sterilizer units of this type are sold, for example, under thebrand name Pondmaster by Danner Manufacturing, Inc. of Islandia N.Y. Forcontinuous treatment processes, the dispersion can be circulated thoughunits of this type to expose the dispersion to ultraviolet light. Singlepass or multiple pass treatments can be employed.

Dispersion can also be processed in a batch operation in a containersuitable for exposure to ultraviolet light in the presence of an oxygensource. In this pretreatment, it is desirable for a suitably protectedultraviolet lamp to be immersed in the dispersion. For example, a vesselnormally used for coagulation of the aqueous fluoropolymer dispersion toproduce fluoropolymer resin can be used for carrying out the process ofthis pretreatment by immersing the ultraviolet lamp in the dispersionheld in this vessel. The dispersion can be circulated or stirred ifdesired to facilitate exposure to the ultraviolet light. When the oxygensource is a gas as discussed below, circulation may be achieved orenhanced by injecting the oxygen source into the dispersion. Ultravioletlamps with protective quartz tubes of the type employed in thesubmersible UV clarifier/sterilizer units can be employed for immersionin dispersion after being removed from their housing. Other ultravioletlamps such as medium-pressure mercury-vapor lamps can also be used withthe lamp suitably protected for immersion in the dispersion such as byenclosing the lamp in a quartz photowell. A borosilicate glass photowellcan also be used although it may decrease effectiveness by filteringultraviolet light in the UVC and UVB bands. Suitable medium-pressuremercury vapor lamps are sold by Hanovia of Fairfield, N.J.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source employed in accordance withthis pretreatment is preferably selected from the group consisting ofair, oxygen rich gas, ozone containing gas and hydrogen peroxide.“Oxygen rich gas” means pure oxygen and gas mixtures containing greaterthan about 21% oxygen by volume, preferably oxygen enriched air.Preferably, oxygen rich gas contains at least about 22% oxygen byvolume. “Ozone containing gas” means pure ozone and gas mixturescontaining ozone, preferably ozone enriched air. Preferably, the contentof ozone in the gas mixture is at least about 10 ppm ozone by volume.

For the practice of this pretreatment, one preferred oxygen source is anozone containing gas. Another preferred oxygen source for the practiceof this pretreatment is hydrogen peroxide. For providing the presence ofthe oxygen source in the dispersion during exposure to ultravioletlight, air, oxygen rich gas or ozone containing gas can be injectedcontinuously or intermittently into the dispersion, preferably instoichiometric excess, to provide the oxygen source during the exposureto ultraviolet light. Hydrogen peroxide can be added to the dispersion,also preferably in stoichiometric excess, by adding hydrogen peroxidesolution. The concentration of hydrogen peroxide is preferably about 0.1weight % to about 10 weight % based on fluoropolymer solids in thedispersion.

Ultraviolet light with an oxygen source is effective at ambient ormoderate temperatures and thus elevated temperatures are typically notrequired for the practice of this pretreatment. In a preferred form ofthis pretreatment, exposing the aqueous fluoropolymer dispersion toultraviolet light in the presence of an oxygen source is carried out ata temperature of about 5° C. to about 70° C., preferably about 15° C. toabout 70° C.

The time for carrying out this pretreatment with vary with factorsincluding the power of the ultraviolet light used, the type of oxygensource, processing conditions, etc. Preferred times for thispretreatment are about 15 minutes to about 10 hours.

Another preferred pretreatment comprises exposing the aqueousfluoropolymer dispersion to light having a wavelength of 10 nm to 760 nmin the presence of an oxygen source and photocatalyst. For the practiceof this pretreatment, the aqueous fluoropolymer dispersion is preferablyfirst diluted with water to a concentration less than the concentrationof the as polymerized aqueous fluoropolymer dispersion because,depending upon the equipment used, exposure to light can be moreeffective for reducing discoloration for dilute dispersions. Preferredconcentrations are about 2 weight percent to about 30 weight %, morepreferably about 2 weight percent to about 20 weight percent.

Light to be employed in accordance with this pretreatment has awavelength range or about 10 nm to about 760 nm. This wavelength rangeincludes ultraviolet light having a wavelength range of about 10 nm toabout 400 nm. Ultraviolet light has a wavelength range or about 10 nm toabout 400 nm and has been described to have bands including: UVA (315 nmto 400 nm), UVB (280 nm to 315 nm), and UVC (100 nm to 280 nm). Light tobe employed in accordance with this pretreatment also includes visiblelight having a wavelength range of about 400 nm to about 760 nm.

Any of various types of lamps can be used as the source of light. Forexample, submersible UV clarifier/sterilizer units sold for the purposesof controlling algae and bacterial growth in ponds are commerciallyavailable and may be used for the practice of this pretreatment. Theseunits include a low-pressure mercury-vapor UVC lamp within a housing forthe circulation of water. The lamp is protected by a quartz tube so thatwater can be circulated within the housing for exposure to ultravioletlight. Submersible UV clarifier/sterilizer units of this type are sold,for example, under the brand name Pondmaster by Danner Manufacturing,Inc. of Islandia N.Y. For continuous treatment processes, the dispersioncan be circulated though units of this type to expose the dispersion tolight. Single pass or multiple pass treatments can be employed.

Dispersion can also be processed in a batch operation in a containersuitable for exposure to light in the presence of an oxygen source andphotocatalyst. In this pretreatment, it is desirable for a suitablyprotected lamp to be immersed in the dispersion. For example, a vesselnormally used for coagulation of the aqueous fluoropolymer dispersion toproduce fluoropolymer resin can be used for carrying out the process ofthis pretreatment by immersing the lamp in the dispersion held in thisvessel. The dispersion can be circulated or stirred if desired tofacilitate exposure to the light. When the oxygen source is a gas asdiscussed below, circulation may be achieved or enhanced by injectingthe oxygen source into the dispersion. Ultraviolet lamps with protectivequartz tubes of the type employed in the submersible UVclarifier/sterilizer units can be employed for immersion in dispersionafter being removed from their housing. Other ultraviolet lamps such asmedium-pressure mercury vapor lamps can also be used with the lampsuitably protected for immersion in the dispersion such as by enclosingthe lamp in a quartz photowell. A borosilicate glass photowell can alsobe used although it may decrease effectiveness by filtering ultravioletlight in the UVC and UVB bands. Suitable medium-pressure mercury vaporlamps are sold by Hanovia of Fairfield, N.J.

As used in this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source employed in accordance with thepresent this pretreatment is preferably selected from the groupconsisting of air, oxygen rich gas, ozone containing gas and hydrogenperoxide. “Oxygen rich gas” means pure oxygen and gas mixturescontaining greater than about 21% oxygen by volume, preferably oxygenenriched air. Preferably, oxygen rich gas contains at least about 22%oxygen by volume. “Ozone containing gas” means pure ozone and gasmixtures containing ozone, preferably ozone enriched air. Preferably,the content of ozone in the gas mixture is at least about 10 ppm ozoneby volume.

For the practice of this pretreatment, one preferred oxygen source is anozone containing gas. Another preferred oxygen source for the practiceof this pretreatment is hydrogen peroxide. For providing the presence ofthe oxygen source in the dispersion during exposure to ultravioletlight, air, oxygen rich gas or ozone containing gas can be injectedcontinuously or intermittently into the dispersion, preferably instoichiometric excess, to provide the oxygen source during the exposureto light. Hydrogen peroxide can be added to the dispersion, alsopreferably in stoichiometric excess, by adding hydrogen peroxidesolution. The concentration of hydrogen peroxide is preferably about 0.1weight ° A to about 10 weight % based on fluoropolymer solids in thedispersion.

Any of a variety of photocatalysts may be used in the practice of thispretreatment. Preferably, the photocatalyst is a heterogeneousphotocatalyst. Most preferably, the heterogeneous photocatalyst isselected from form the group consisting of titanium dioxide and zincoxide. For example, titanium dioxide sold under the tradename DegussaP25 having a primary particle size of 21 nm and being a mixture of 70%anatase and 30% rutile titanium dioxide has been found to be aneffective heterogeneous photocatalyst. Heterogeneous photocatalyst canbe used by dispersing it into the dispersion prior to exposure to light.Preferred levels of heterogenous photocatalyst are about 1 ppm to about100 ppm based on fluoropolymer solids in the dispersion.

Light with an oxygen source and photocatalyst is effective at ambient ormoderate temperatures and thus elevated temperatures are typically notrequired for the practice of this pretreatment. In a preferred processin accordance with this pretreatment, exposing the aqueous fluoropolymerdispersion to ultraviolet light in the presence of an oxygen source iscarried out at a temperature of about 5° C. to about 70° C., preferablyabout 15° C. to about 70° C.

The time for carrying out this pretreatment will vary with factorsincluding the power of the ultraviolet light used, the type of oxygensource, processing conditions, etc. Preferred times for thispretreatment are about 15 minutes to about 10 hours.

Another preferred pretreatment comprises exposing the aqueousfluoropolymer dispersion to hydrogen peroxide. For the practice of thispretreatment, the aqueous fluoropolymer dispersion is preferably firstdiluted with water to a concentration less than the concentration of theas polymerized aqueous fluoropolymer dispersion. Preferredconcentrations are about 2 weight percent to about 30 weight percent,more preferably about 2 weight percent to about 20 weight percent.

Exposing of the aqueous fluoropolymer dispersion to hydrogen peroxide ispreferably carried out by adding hydrogen peroxide to said aqueousfluoropolymer dispersion, preferably in an amount of about 0.1 weight %to about 10 weight percent based on weight of fluoropolymer solids.Preferably, the exposing of the aqueous fluoropolymer dispersion tohydrogen peroxide is carried out at a temperature of about 10° C. toabout 70° C., preferably about 25° C. to about 60° C. The time employedfor the exposure of the aqueous fluoropolymer dispersion is preferablyabout 1 hour to about 48 hours.

It is preferable for the practice of this pretreatment to also injectair, oxygen rich gas, or ozone containing gas into said fluoropolymerdispersion during the exposing of the aqueous fluoropolymer dispersionto the hydrogen peroxide. “Oxygen rich gas” means pure oxygen and gasmixtures containing greater than about 21% oxygen by volume, preferablyoxygen enriched air. Preferably, oxygen rich gas contains at least about22% oxygen by volume. “Ozone containing gas” means pure ozone and gasmixtures containing ozone, preferably ozone enriched air. Preferably,the content of ozone in the gas mixture is at least about 10 ppm ozoneby volume. Introduction of such gases can be accomplished by injectingthe gases into the aqueous fluoropolymer dispersion.

Preferably, the exposing of the aqueous fluoropolymer dispersion tohydrogen peroxide is carried out in the presence of Fe⁺², Cu⁺¹, or Mn⁺²ions. Preferably, the amount of Fe⁺², Cu⁺¹, or Mn⁺² ions is about 0.1ppm to about 100 ppm based on fluoropolymer solids in the dispersion.

Although the process can also be carried out in a continuous process,batch processes are preferable since batch processes facilitatecontrolled times for exposure of the hydrogen peroxide with the aqueousfluoropolymer dispersion to achieve the desired reduction in thermallyinduced discoloration. A batch process can be carried out in anysuitable tank or vessel of appropriate materials of construction and, ifdesired, has heating capability to heat the dispersion during treatment.For example, a batch process can be carried out in a vessel normallyused for coagulation of the aqueous fluoropolymer dispersion whichtypically includes an impeller which can be used to stirring thedispersion during treatment. Injection of air, oxygen rich gas, or ozonecontaining gas can also be employed to impart agitation to thedispersion.

Another preferred pretreatment comprises exposing the aqueousfluoropolymer dispersion to oxidizing agent selected from the groupconsisting of hypochlorite salts and nitrite salts. For the practice ofthis pretreatment, the aqueous fluoropolymer dispersion is preferablyfirst diluted with water to a concentration less than the concentrationof the as polymerized aqueous fluoropolymer dispersion. Preferredconcentrations are about 2 weight percent to about 30 weight percent,more preferably about 2 weight percent to about 20 weight percent.

Exposing of the aqueous fluoropolymer dispersion to oxidizing agentselected from the group consisting of hypochlorite salts and nitritesalts is preferably carried out by adding the oxidizing agent to theaqueous fluoropolymer dispersion, preferably in an amount of about 0.05weight % to about 5 weight percent based on weight of fluoropolymersolids. Preferred hypochlorite salts for addition to the dispersion aresodium hypochlorite or potassium hypochlorite. Sodium hypochlorite orpotassium hypochlorite are preferably used in an amount of about 0.05weight % to about 5 weight percent based on weight of fluoropolymersolids. Provided that aqueous medium of the dispersion is sufficientlyalkaline such as by containing sodium hydroxide, hypochlorite can alsobe generated in situ by injecting chlorine gas into the dispersion.Preferred nitrite salts for addition to the dispersion are sodiumnitrite, potassium nitrite and ammonium nitrite. Sodium nitrite,potassium nitrite and ammonium nitrite are preferably used in an amountof about 0.5 weight % to about 5 weight percent based on weight offluoropolymer solids.

Preferably, the exposing of the aqueous fluoropolymer dispersion to theoxidizing agent is carried out at a temperature of about 10° C. to about70° C. The exposure time with the aqueous fluoropolymer dispersion ispreferably about 5 minutes to about 3 hours.

It is preferable for the practice of this pretreatment to also introduceair, oxygen rich gas, or ozone containing gas into said fluoropolymerdispersion during the exposing of the aqueous fluoropolymer dispersionto the oxidizing agent. “Oxygen rich gas” means pure oxygen and gasmixtures containing greater than about 21% oxygen by volume, preferablyoxygen enriched air. Preferably, oxygen rich gas contains at least about22% oxygen by volume. “Ozone containing gas” means pure ozone and gasmixtures containing ozone, preferably ozone enriched air. Preferably,the content of ozone in the gas mixture is at least about 10 ppm ozoneby volume. Introduction of such gases can be accomplished by injectingsuch gases into the aqueous fluoropolymer dispersion.

Although the pretreatment can also be carried out in a continuousprocess, batch processes are preferable since batch processes facilitatecontrolled times for exposure of the hypochlorite salt or nitrite saltwith the aqueous fluoropolymer dispersion to achieve the desiredreduction in thermally induced discoloration. A batch process can becarried out in any suitable tank or vessel of appropriate materials ofconstruction and, if desired, has heating capability to heat thedispersion during treatment. For example, a batch process can be carriedout in a vessel normally used for coagulation of the aqueousfluoropolymer dispersion which typically includes an impeller which canbe used to stirring the dispersion during treatment. Injection of air,oxygen rich gas, or ozone containing gas can also be employed to impartagitation to the dispersion.

Another preferred pretreatment comprises adjusting the pH of the aqueousmedium of the aqueous fluoropolymer dispersion to greater than about 8.5and exposing the aqueous fluoropolymer dispersion to an oxygen source.For the practice of this pretreatment, the aqueous fluoropolymerdispersion is preferably first diluted with water to a concentrationless than the concentration of the as polymerized aqueous fluoropolymerdispersion. Preferred concentrations are about 2 weight percent to about30 weight percent, more preferably about 2 weight percent to about 20weight percent.

The pH of the aqueous fluoropolymer dispersion preferably is adjusted toabout 8.5 to about 11. More preferably, the pH of the aqueous medium ofthe aqueous fluoropolymer dispersion is adjusted to about 9.5 to about10.

The pH can be adjusted for the practice of this pretreatment by additionof a base which is sufficiently strong to adjust the pH of the aqueousfluoropolymer dispersion to the desired level and which is otherwisecompatible with the processing of the dispersion and the end useproperties of the fluoropolymer resin produced. Preferred bases arealkali metal hydroxides such as sodium hydroxide or potassium hydroxide.Ammonium hydroxide can also be used.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source employed in accordance withthis pretreatment is preferably selected from the group consisting ofair, oxygen rich gas, ozone containing gas and hydrogen peroxide.“Oxygen rich gas” means pure oxygen and gas mixtures containing greaterthan about 21% oxygen by volume, preferably oxygen enriched air.Preferably, oxygen rich gas contains at least about 22% oxygen byvolume. “Ozone containing gas” means pure ozone and gas mixturescontaining ozone, preferably ozone enriched air. Preferably, the contentof ozone in the gas mixture is at least about 10 ppm ozone by volume.

For the practice of this pretreatment, one preferred oxygen source is anozone containing gas. Another preferred oxygen source is for thepractice of this pretreatment is hydrogen peroxide. For providing theexposure of dispersion to the oxygen source, air, oxygen rich gas orozone containing gas can be injected continuously or intermittently intothe dispersion, preferably in stoichiometric excess. Hydrogen peroxidecan be added to the dispersion, also preferably in stoichiometricexcess, by adding hydrogen peroxide solution. The concentration ofhydrogen peroxide is preferably about 0.1 weight % to about 10 weight %based on fluoropolymer solids in the dispersion.

Preferably, the exposing of the aqueous fluoropolymer dispersion tooxygen source is carried out at a temperature of about 10° C. to about95° C., more preferably about 20° C. to about 80° C., most preferablyabout 25° C. to about 70° C. The time employed for the exposure of theaqueous fluoropolymer dispersion to oxygen source is preferably about 5minutes to about 24 hours.

Although the process can also be carried out in a continuous process,batch processes are preferable since batch processes facilitatecontrolled times for exposure of the hydrogen peroxide with the aqueousfluoropolymer dispersion to achieve the desired reduction in thermallyinduced discoloration. A batch process can be carried out in anysuitable tank or vessel of appropriate materials of construction and, ifdesired, has heating capability to heat the dispersion during treatment.For example, a batch process can be carried out in a vessel normallyused for coagulation of the aqueous fluoropolymer dispersion whichtypically includes an impeller which can be used to stirring thedispersion during treatment. Injection of air, oxygen rich gas, or ozonecontaining gas can also be employed to impart agitation to thedispersion.

Another preferred pretreatment comprises heating the fluoropolymer to atemperature of about 160° C. to about 400° C. and exposing the heatedfluoropolymer resin to an oxygen source. In one embodiment of thispretreatment, heating of the fluoropolymer is carried out by convectionheating such as in an oven. Preferably, heat transfer gas employed inthe oven is the oxygen source or includes the oxygen source as will bediscussed below. The heat transfer gas may be circulated to improve heattransfer if desired and the heat transfer gas may include water vapor toincrease its humidity.

This pretreatment is advantageously employed for fluoropolymer resinwhich is melt-processible. The process can be carried out with amelt-processible fluoropolymer resin heated to below or above themelting point of the fluoropolymer resin. Preferably, the process for amelt-processible resin is carried out with the fluoropolymer resinheated to above its melting point.

This pretreatment is also advantageously employed for PTFE fluoropolymerresins (including modified PTFE resins) which are not melt-processible.It is preferred for PTFE resins to be processed below their meltingpoint. Most preferably, PTFE resins are heated to a temperature lessthan 200° C.

The fluoropolymer can be in various physical forms for processing inaccordance with this pretreatment. For processing below the meltingpoint of the fluoropolymer resin, the physical form of the fluoropolymerwill have a greater impact on the time necessary to achieve a desiredreduction in thermally induced discoloration. Preferably for processingbelow the melting point, the fluoropolymer resin is processed in finelydivided form to promote exposure to the oxygen source such as byemploying the powder recovered from isolation of the fluoropolymer, alsocalled flake, prior to melt processing into chip or pellet. Forprocessing above the melting point, the physical form of thefluoropolymer resin is usually less important since the fluoropolymerresin will melt and fuse when heating. Although chip or pellet can alsobe used for treatment above the melting point, the powder recovered fromisolation of the fluoropolymer prior to melt processing into chip orpellet is suitably used. The fluoropolymer resin can be in wet or dryform. If wet fluoropolymer resin is used, drying of the wetfluoropolymer resin results as it is heated.

For this pretreatment, the fluoropolymer resin can be contained in anopen container of suitable material such as aluminum, stainless steel,or high nickel alloy such as that sold under the trademark Monel®.Preferably, pans or trays are employed which have a shallow depth topromote exposure to and mass transfer of oxygen from the oxygen sourceinto the fluoropolymer resin.

The pretreatment can be carried out such that the fluoropolymer resin isunder static conditions or dynamic conditions. The process is preferablycarried out with the fluoropolymer resin under static conditions if thefluoropolymer is processed above the melting point and is preferablycarried out with the fluoropolymer resin under dynamic conditions ifprocessed below the melting point. “Static conditions” means that thefluoropolymer is not subjected to agitation such as by stirring orshaking although the heat transfer gas for convection heating may becirculated as noted above. Under static conditions, some settling of theresin may occur or, if conducted above the melting point, some flow ofthe melted resin within the container may occur. “Dynamic conditions”means that the process is carried while moving the fluoropolymer resinsuch as by stirring or shaking or actively passing a heat transfer gasthrough the fluoropolymer resin which may additionally cause movementthe fluoropolymer resin. Heat transfer and mass transfer can befacilitated by the use of dynamic conditions which can be provided by,for example, a fluidized bed reactor or by otherwise flowing the gasthrough the polymer bed.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. The oxygen source preferably is either the heattransfer gas or is a component of the heat transfer gas. Preferably, theoxygen source is air, oxygen rich gas, or ozone-containing gas. “Oxygenrich gas” means pure oxygen and gas mixtures containing greater thanabout 21% oxygen by volume, preferably oxygen enriched air. Preferably,oxygen rich gas contains at least about 22% oxygen by volume. “Ozonecontaining gas” means pure ozone and gas mixtures containing ozone,preferably ozone enriched air. Preferably, the content of ozone in thegas mixture is at least about 10 ppm ozone by volume. For example, whenthe oxygen source is air, an air oven can be used to carry out theprocess. Oxygen or ozone can be supplied to the air oven to provide anoxygen rich gas, i.e., oxygen enriched air, or ozone-containing gas,i.e., ozone enriched air, respectively.

The time necessary to carry out this pretreatment will vary with factorsincluding the temperature employed, the oxygen source employed, the rateof circulation of the heat transfer gas, and the physical form of thefluoropolymer resin. In general, treatment times for the process carriedout below the melting point of the fluoropolymer are significantlylonger than those for processes carried out above the melting point. Forexample, fluoropolymer resin treated using air as the oxygen sourcebelow the melting point may require processing for about 1 to 25 days toachieve the desired color reduction. The time for a process carried outusing air as the oxygen source above the melting point generally mayvary from about 15 minutes to about 10 hours.

Resin treated above the melting point typically results in the formationof solid slabs of fluoropolymer resin which may be chopped intosuitably-sized pieces to feed a melt extruder for subsequent processing.

Another preferred pretreatment comprises melt extruding thefluoropolymer resin to produce molten fluoropolymer resin and exposingthe molten fluoropolymer resin to an oxygen source during the meltextruding. “Melt extruding” as used for this pretreatment means to meltthe fluoropolymer resin and to subject the molten fluoropolymer resin tomixing of the fluoropolymer resin. Preferably, the melt extrudingprovides sufficient shear to provide effective exposure of the oxygensource with the molten fluoropolymer resin. To carry out melt extrusionfor this pretreatment, various equipment can be used. Preferably, themolten fluoropolymer resin is processed in a melt extruder.Fluoropolymer flake after isolation is often processed by melt extrusioninto chip or pellet and this is a convenient point in the manufacturingprocess to practice the process of this pretreatment. Various types ofextruders such a single-screw or multi-screw extruder can be used.Combinations of extruders are also suitably used. Preferably, the meltextruder provides a high shear section such as by including kneadingblock sections or mixing elements to impart high shear to the moltenfluoropolymer resin. In the event more residence time than can beprovided in an extruder is desired, a kneader such as a surface renewaltype kneader as disclosed in Hiraga et al. U.S. Pat. No. 6,664,337 canbe used to carry out this pretreatment.

For the practice of the process of this pretreatment, extruders orkneaders are suitably fitted with a port or ports for injecting theoxygen source for exposure with the fluoropolymer. A vacuum port forremoving volatiles is also preferably provided. Equipment and methodsuseful for stabilizing melt-processible fluoropolymers, for example,those disclosed in Chapman et al., U.S. Pat. No. 6,838,545, can be usedto carry out the process of this pretreatment.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. Preferably, the oxygen source is air, oxygen richgas, or ozone-containing gas. “Oxygen rich gas” means pure oxygen andgas mixtures containing greater than about 21% oxygen by volume,preferably oxygen enriched air. Preferably, oxygen rich gas contains atleast about 22% oxygen by volume. “Ozone containing gas” means pureozone and gas mixtures containing ozone, preferably ozone enriched air.Preferably, the content of ozone in the gas mixture is at least about 10ppm ozone by volume.

In the practice of this pretreatment, the oxygen source can be injectedto an appropriate port in the melt extruding equipment and the moltenfluoropolymer resin is thereby exposed to the oxygen source. Thelocation at which the molten polymer is exposed to oxygen source may bereferred to as the reaction zone. In preferred melt extruders for thepractice of this pretreatment having at least one high shear sectionprovided with kneading blocks or mixing elements, the moltenfluoropolymer resin is exposed to the oxygen source in the high shearsection, i.e., the reaction zone is in a high shear section. Preferably,the process of this pretreatment is carried out in multiple stages,i.e., the extruder has more than one reaction zone for exposure of themolten fluoropolymer to oxygen source. The amount of oxygen sourcerequired will vary with the degree of thermally induced discolorationexhibited by the fluoropolymer resin. It is usually desirable to employa stoichiometric excess of the oxygen source.

Another preferred pretreatment comprises exposing wet fluoropolymerresin to an oxygen source during drying. The wet fluoropolymer resin foruse in this pretreatment is preferably undispersed fluoropolymer asseparated from the dispersion. Any of various equipment known for use indrying fluoropolymer resin can be used for this pretreatment. In suchequipment a heated drying gas, typically air, is used as a heat transfermedium to heat the fluoropolymer resin and to convey away water vaporand chemicals removed from the fluoropolymer resin during drying.Preferably in accordance with this pretreatment, the drying gas employedis the oxygen source or includes the oxygen source as discussed below.

The process of this pretreatment can be carried out such that thefluoropolymer resin is dried under static conditions or dynamicconditions. “Static conditions” means that the fluoropolymer is notsubjected to agitation such as by stirring or shaking during dryingalthough drying in equipment such as tray drying in an oven result incirculation of the drying gas by convection. “Dynamic conditions” meansthat the process is carried while moving the fluoropolymer resin such asby stirring or shaking or actively passing a drying gas through thefluoropolymer resin which may additionally cause movement thefluoropolymer resin. Heat transfer and mass transfer can be facilitatedby the use of dynamic conditions, for example, flowing the drying gasthrough the polymer bed. Preferably, the process of this pretreatment iscarried out under dynamic conditions. Preferred equipment and processconditions for drying under dynamic conditions is disclosed by Egres,Jr. et al. U.S. Pat. No. 5,391,709, in which the wet fluoropolymer resinis deposited as a shallow bed on fabric and dried by passing heated airthrough the bed, preferably from top to bottom.

As used for this pretreatment, “oxygen source” means any chemical sourceof available oxygen. “Available oxygen” means oxygen capable of reactingas an oxidizing agent. Preferably, the oxygen source is air, oxygen richgas, or ozone-containing gas. “Oxygen rich gas” means pure oxygen andgas mixtures containing greater than about 21% oxygen by volume,preferably oxygen enriched air. Preferably, oxygen rich gas contains atleast about 22% oxygen by volume. “Ozone containing gas” means pureozone and gas mixtures containing ozone, preferably ozone enriched air.Preferably, the content of ozone in the gas mixture is at least about 10ppm ozone by volume.

One preferred oxygen source for practice of this pretreatment is ozonecontaining gas, preferably ozone enriched air. Ozone enriched air as thedrying gas can be provided by employing an ozone generator which feedsozone into the drying air as it is supplied to the drying apparatusused. Another preferred oxygen source is oxygen rich gas, preferablyoxygen enriched air. Oxygen enriched air as the drying gas can beprovided by feeding oxygen into the drying air as it is supplied to thedrying apparatus used. Oxygen enriched air can also be provided bysemipermeable polymeric membrane separation systems.

Temperatures of drying gas during drying can be in the range of about100° C. to about 300° C. Higher temperature drying gases shorten thedrying time and facilitate the reduction of thermally induceddiscoloration. However, temperatures of the drying gas should not causethe temperature of the fluoropolymer resin to reach or exceed itsmelting point which will cause the fluoropolymer to fuse. Formelt-processible fluoropolymers, preferred drying gas temperatures are160° C. to about 10° C. below the melting point of the fluoropolymer.The end use properties of PTFE resin can be adversely affected bytemperatures well below its melting point. Preferably, PTFE resin isdried using drying gas at a temperature of about 100° C. to about 200°C., more preferably, about 150° C. to about 180° C.

The time necessary to carry out the process of this pretreatment willvary with factors including the thickness of the wet fluoropolymer resinbeing dried, the temperature employed, the oxygen source employed andthe rate of circulation of the drying gas. When ozone containing gas isused as the oxygen source, the reduction of thermally induceddiscoloration can be accomplished during normal drying times, preferablyin the range of about 15 minutes to 10 hours. If desired, thepretreatment can be continued after the fluoropolymer resin is dry forthe purposes of reducing thermally induced discoloration.

More than one pretreatment can be employed if desired and suchpretreatments can be performed on aqueous fluoropolymer dispersion,fluoropolymer resin or both.

Exposure to fluorine may be carried out with a variety of fluorineradical generating compounds but preferably exposure of thefluoropolymer resin is carried out by contacting the fluoropolymer resinwith fluorine gas. Since the reaction with fluorine is very exothermic,it is preferred to dilute the fluorine with an inert gas such asnitrogen. The level of fluorine in the fluorine/inert gas mixture may be1 to 100 volume % but is preferably about 5 to about 25 volume % becauseit is more hazardous to work with pure fluorine. For fluoropolymerresins in which the thermally induced discoloration is severe, thefluorine/inert gas mixture should be sufficiently dilute to avoidoverheating the fluoropolymer and the accompanying risk of fire.

Heating the fluoropolymer resin during exposure to fluorine increasesthe reaction rate. Because the reaction of fluorine to reduce thermallyinduced discoloration is very exothermic, some or all of the desiredheating may be provided by the reaction with fluorine. The process ofthe invention can be carried out with the fluoropolymer resin heated toa temperature above the melting point of the fluoropolymer resin or at atemperature below the melting point of the fluoropolymer resin.

For the process carried out below the melting point, the exposing of thefluoropolymer resin to fluorine is preferably carried out with thefluoropolymer resin heated to a temperature of about 20° C. to about250° C. In one embodiment, the temperature employed is about 150° C. toabout 250° C. In one another embodiment, the temperature is about 20° C.to about 100° C. For PTFE fluoropolymer resins (including modified PTFEresins) which are not melt-processible, i.e., PTFE fine powders, it isdesirable to carry the process below the melting point of the PTFE resinto avoid sintering and fusing the resin. Preferably, PTFE fine powderresins are heated to a temperature less than about 200° C. to avoidadversely affecting end use characteristics of the PTFE resin. In onepreferred embodiment, the temperature is about 20° C. to about 100° C.

For fluoropolymers which are melt-processible, the process can becarried out with the fluoropolymer heated to below or above the meltingpoint of the fluoropolymer resin. Preferably, the process for amelt-processible resin is carried out with the fluoropolymer resinheated to above its melting point. Preferably, the exposing of thefluoropolymer resin to fluorine is carried out with the fluoropolymerresin heated to a temperature above its melting up to about 400° C.

For processing with the fluoropolymer resin heated to below the meltingpoint, the fluoropolymer resin is preferably processed in particulateform to provide desirable reaction rates such as powders, flake, pelletsor beads. Suitable apparatus for processing below the melting point aretanks or vessels which contain the fluoropolymer resin for exposure to afluorine or fluorine/inert gas mixture while stirring, tumbling, orfluidizing the fluoropolymer resin for uniform exposure of the resin tofluorine. For example, a double cone blender can be used for thispurpose. Equipment and methods useful for the removal of unstable endgroups in melt-processible fluoropolymers, for example, those disclosedin Morgan et al., U.S. Pat. No. 4,626,587 and Imbalzano et al., U.S.Pat. No. 4,743,658, can be used to expose the fluoropolymer resin tofluorine at a temperature below its melting point. In general, morefluorine is necessary for reducing thermally induced discoloration todesirable level than is typically required for removing unstable endgroups, for example, at least 2 times the amount required for removingunstable end groups can be required. The amount of fluorine requiredwill be dependent upon the level of discoloration but it is usuallydesirable to employ a stoichiometric excess of fluorine.

For processing the fluoropolymer resin heated to above the meltingpoint, exposure to fluorine can be accomplished by a variety of methodswith reactive extrusion being a preferred method for the practice of thepresent invention. In reactive extrusion, exposure to fluorine isperformed while the molten polymer is processed in a melt extruder. Whenfluoropolymer flake is processed by melt extrusion into chip or pelletis a convenient point in the manufacturing process to practice theprocess of the invention. Various types of extruders such a single-screwor multi-screw extruders can be used. Combinations of extruders are alsosuitably used. Preferably, the extruder includes mixing elements toimprove mass transfer between the gas and the molten fluoropolymerresin. For the practice of the process of the invention, extruders aresuitably fitted with a port or ports for feeding fluorine orfluorine/inert gas mixture for contacting the fluoropolymer. A vacuumport for removing volatiles is also preferably provided. Equipment andmethods useful for stabilizing melt-processible fluoropolymers, forexample, those disclosed in Chapman et al., U.S. Pat. No. 6,838,545,Example 2, can be used to expose the fluoropolymer to fluorine at atemperature above its melting point. Similar to the process carried outbelow the melting point, more fluorine is generally necessary forreducing thermally induced discoloration to desirable level than istypically required for removing unstable end groups, for example, atleast 2 times the amount required for removing unstable end groups canbe required. The amount of fluorine required will be dependent upon thelevel of discoloration, but it is usually desirable to employ astoichiometric excess of fluorine. In the event more residence time thanis provided in an extruder is desired for the exposure to fluorine, akneader such as a surface renewal type kneader as disclosed in Hiraga etal. U.S. Pat. No. 6,664,337 can be used to carry out the process of theinvention.

The process of the invention is useful for fluoropolymer resin whichexhibits thermally induced discoloration which may range from mild tosevere. The process is especially useful for aqueous fluoropolymerdispersion which contains hydrocarbon surfactant which causes thethermally induced discoloration, preferably aqueous fluoropolymerdispersion that is polymerized in the presence of hydrocarbonsurfactant.

The process of the invention is especially useful when the fluoropolymerresin prior to treatment exhibits significant thermally induceddiscoloration compared to equivalent commercial fluoropolymers. Theinvention is advantageously employed when the fluoropolymer resin has aninitial thermally induced discoloration value (L*_(i)) at least about 4L units below the L* value of equivalent fluoropolymer resin ofcommercial quality manufactured using ammonium perfluorooctanoatefluorosurfactant. The invention is more advantageously employed when theL*_(i) value is at least about 5 units below the L* value of suchequivalent fluoropolymer resin, even more advantageously employed whenthe L*_(i) value is at least 8 units below the L* value of suchequivalent fluoropolymer resin, still more advantageously employed whenthe L*_(i) value is at least 12 units below the L* value of suchequivalent fluoropolymer resin, and most advantageously employed whenthe L*_(i) value is at least 20 units below the L* value of suchequivalent fluoropolymer resin.

After the fluoropolymer resin is treated in accordance with the processof the invention, the resulting fluoropolymer resin is suitable for enduse applications appropriate for the particular type of fluoropolymerresin. Fluoropolymer resin produced by employing the present inventionexhibits reduced thermally induced discoloration without detrimentaleffects on end use properties.

Test Methods

Raw Dispersion Particle Size (RDPS) of polymer particles is measuredusing a Zetasizer Nano-S series dynamic light scattering systemmanufactured by Malvern Instruments of Malvern, Worcestershire, UnitedKingdom. Samples for analysis are diluted to levels recommended by themanufacturer in 10×10×45 mm polystyrene disposable cuvettes usingdeionized water that has been rendered substantially free of particlesby passing it through a sub-micron filter. The sample is placed in theZetasizer for determination of Dv(50). Dv(50) is the median particlesize based on volumetric particle size distribution, i.e. the particlesize below which 50% of the volume of the population resides.

The melting point (T_(m)) of melt-processible fluoropolymers is measuredby Differential Scanning calorimeter (DSC) according to the procedure ofASTM D 4591-07 with the melting temperature reported being the peaktemperature of the endotherm of the second melting. For PTFEhomopolymer, the melting point is also determined by DSC. The unmeltedPTFE homopolymer is first heated from room temperature to 380° C. at aheating rate of 10° C. and the melting temperature reported is the peaktemperature of the endotherm on first melting.

Comonomer content is measured using a Fourier Transform Infrared (FTIR)spectrometer according to the method disclosed in U.S. Pat. No.4,743,658, col. 5, lines 9-23 with the following modifications. The filmis quenched in a hydraulic press maintained at ambient conditions. Thecomonomer content is calculated from the ratio of the appropriate peakto the fluoropolymer thickness band at 2428 cm⁻¹ calibrated using aminimum of three other films from resins analyzed by fluorine 19 NMR toestablish true comonomer content. For instance, the % HFP content isdetermined from the absorbance of the HFP band at 982 cm⁻¹, and the PEVEcontent is determined by the absorbance of the PEVE peak at 1090 cm⁻¹.

Melt flow rate (MFR) of the melt-processible fluoropolymers are measuredaccording to ASTM D 1238-10, modified as follows: The cylinder, orificeand piston tip are made of a corrosion-resistant alloy, Haynes Stellite19, made by Haynes Stellite Co. The 5.0 g sample is charged to the 9.53mm (0.375 inch) inside diameter cylinder, which is maintained at 372°C.±1° C., such as disclosed in ASTM D 2116-07 for FEP and ASTM D 3307-10for PFA. Five minutes after the sample is charged to the cylinder, it isextruded through a 2.10 mm (0.0825 inch) diameter, 8.00 mm (0.315 inch)long square-edge orifice under a load (piston plus weight) of 5000grams. Other fluoropolymers are measured according to ASTM D 1238-10 atthe conditions which are standard for the specific polymer.

Measurement of Thermally Induced Discoloration

1) Color Determination

The L* value of fluoropolymer resin samples is determined using theCIELAB color scale, details of which are published in CIE Publication15.2 (1986). CIE L*a*b* (CIELAB) is the color space specified by theInternational Commission on Illumination (French Commissioninternationale de l'éclairage). It describes all the colors visible tothe human eye. The three coordinates of CIELAB represent the lightnessof the color (L*), its position between red/magenta and green (a*), andits position between yellow and blue (b*).

2) PTFE Sample Preparation and Measurement

The following procedure is used to characterize the thermally induceddiscoloration of PTFE polymers including modified PTFE polymers. 4.0gram chips of compressed PTFE powder are formed using a Carver stainlesssteel pellet mold (part #2090-0) and a Carver manual hydraulic press(model 4350), both manufactured by Carver, Inc. of Wabash, Ind. In thebottom of the mold assembly is placed a 29 mm diameter disk of 0.1 mmthick Mylar film. 4 grams of dried PTFE powder are spread uniformlywithin the mold opening poured into the mold and distributed evenly. Asecond 29 mm disk is placed on top of the PTFE and the top plunger isplaced in the assembly. The mold assembly is placed in the press andpressure is gradually applied until 8.27 MPa (1200 psi) is attained. Thepressure is held for 30 seconds and then released. The chip mold isremoved from the press and the chip is removed from the mold. Mylarfilms are pealed from the chip before subsequent sintering. Typicallyfor each polymer sample, two chips are molded.

An electric furnace is heated is heated to 385° C. Chips to be sinteredare placed in 4 inch×5 inch (10.2 cm×12.7 cm) rectangular aluminum trayswhich are 2 inches (5.1 cm) in depth. The trays are placed in thefurnace for 10 minutes after which they are removed to ambienttemperature for cooling.

4 gm chips processed as described above are evaluated for color using aHunterLab Color Quest XE made by Hunter Associates Laboratory, Inc. ofReston, Va. The Color Quest XE sensor is standardized with the followingsettings, Mode: RSIN, Area View: Large and Port Size: 2.54 cm. Theinstrument is used to determine the L* value of fluoropolymer resinsamples using the CIELAB color scale.

For testing, the instrument is configured to use CIELAB scale with D65Illuminant and 10° Observer. The L* value reported by this colorimeteris used to represent developed color with L* of 100 indicating a perfectreflecting diffuser (white) and L* of 0 representing black.

An equivalent fluoropolymer resin of commercial quality manufacturedusing ammonium perfluorooctanoate fluorosurfactant is used as thestandard for color measurements. For the Examples in this applicationillustrating the invention for PTFE fluoropolymer, an equivalentcommercial quality PTFE product made using ammonium perfluorooctanoatefluorosurfactant as the dispersion polymerization surfactant is TEFLON®601A. Using the above measurement process, the resulting colormeasurement for TEFLON® 601A is L*_(Std-PTFE)=87.3

3) Melt-Processible Fluoropolymers Sample Preparation and Measurement

The following procedure is used to characterize discoloration ofmelt-processible fluoropolymers, such as FEP and PFA, upon heating. A10.16 cm (4.00 inch) by 10.16 cm (4.00 inch) opening is cut in themiddle of a 20.32 cm (8.00 inch) by 20.32 cm (8.00 inch) by 0.254 mm(0.010 inch) thick metal sheet to form a chase. The chase is placed on a20.32 cm (8.00 inch) by 20.32 cm (8.00 inch) by 1.59 mm ( 1/16 inch)thick molding plate and covered with Kapton® film that is slightlylarger than the chase. The polymer sample is prepared by reducing size,if necessary, to no larger than 1 mm thick and drying. 6.00 grams ofpolymer sample is spread uniformly within the mold opening. A secondpiece of Kapton® film that is slightly larger than the chase is placedon top of the sample and a second molding plate, which has the samedimensions as the first, is placed on top of the Kapton® film to form amold assembly. The mold assembly is placed in a P-H-I 20 ton hot pressmodel number SP-210C-X4A-21 manufactured by Pasadena HydraulicsIncorporated of El Monte, Calif. that is set at 350° C. The hot press isclosed so the plates are just contacting the mold assembly and held for5 minutes. The pressure on the hot press is then increased to 34.5 MPa(5,000 psi) and held for an additional 1 minute. The pressure on the hotpress is then increased from 34.5 MPa (5,000 psi) to 137.9 MPa (20,000psi) over the time span of 10 seconds and held for an additional 50seconds after reaching 137.9 MPa (20,000 psi). The mold assembly isremoved from the hot press, placed between the blocks of a P-H-I 20 tonhot press model number P-210H manufactured by Pasadena HydraulicsIncorporated that is maintained at ambient temperature, the pressure isincreased to 137.9 MPa (20,000 psi), and the mold assembly is left inplace for 5 minutes to cool. The mold assembly is then removed from theambient temperature press, and the sample film is removed from the moldassembly. Bubble-free areas of the sample film are selected and 2.86 cm(1⅛ inch) circles are stamped out using a 1⅛ inch arch punchmanufactured by C. S. Osborne and Company of Harrison, N.J. Six of thefilm circles, each of which has a nominal thickness of 0.254 mm (0.010inch) and nominal weight of 0.37 gram are assembled on top of each otherto create a stack with a combined weight of 2.2+/−0.1 gram.

The film stack is placed in a HunterLab ColorFlex spectrophotometer madeby Hunter Associates Laboratory, Inc. of Reston, Va., and the L* ismeasured using a 2.54 cm (1.00 inch) aperture and the CIELAB scale withD65 Illuminant and 10° Observer.

An equivalent fluoropolymer resin of commercial quality manufacturedusing ammonium perfluorooctanoate fluorosurfactant is used as thestandard for color measurements. For the Examples in this applicationillustrating the invention for FEP fluoropolymer resin, an equivalentcommercial quality FEP resin made using ammonium perfluorooctanoatefluorosurfactant as the dispersion polymerization surfactant is DuPontTEFLON® 6100 FEP. Using the above measurement process, the resultingcolor measurement for DuPont TEFLON® 6100 FEP is L*_(Std-FEP)=79.7.

4) % change in L* with respect to the standard is used to characterizethe change in thermally induced discoloration of the fluoropolymer resinafter treatment as defined by the following equation% change in L*=(L* _(t) −L* _(i))/(L* _(Std) −L* _(i))×100

-   L*_(i)=Initial thermally induced discoloration value, the measured    value for L on the CIELAB scale for fluoropolymer resins prior to    treatment to reduce thermally induced discoloration measured using    the disclosed test method for the type of fluoropolymer.-   L*_(t)=Treated thermally induced discoloration value, the measured    value for L on the CIELAB scale for fluoropolymer resins after    treatment to reduce thermally induced discoloration measured using    the disclosed test method for the type of fluoropolymer.-   Standard for PTFE: measured L*_(Std-PTFE)=87.3-   Standard for FEP: measured V_(Std-FEP)=79.7

EXAMPLES

Fluoropolymer Preparation

FEP-1: Preparation of Hydrocarbon Stabilized TFE/HFP/PEVE Dispersion

A cylindrical, horizontal, water-jacketed, paddle-stirred, stainlesssteel reactor having a length to diameter ratio of about 1.5 and a watercapacity of 10 gallons (37.9 L) is charged with 60 pounds (27.2 kg) ofdeionized water. The reactor temperature then is increased to 103° C.while agitating at 46 rpm. The agitator speed is reduced to 20 rpm andthe reactor is vented for 60 seconds. The reactor pressure is increasedto 15 psig (205 kPa) with nitrogen. The agitator speed is increased to46 rpm while cooling to 80° C. The agitator speed is reduced to 20 rpmand a vacuum is pulled to 12.7 psi (87.6 kPa). A solution containing 500ml of deaerated deionized water, 0.5 grams of Pluronic® 31R1 solutionand 0.3 gm of sodium sulfite is drawn into the reactor. With the reactorpaddle agitated at 20 rpm, the reactor is heated to 80° C., evacuatedand purged three times with TFE. The agitator speed is increased to 46rpm and the reactor temperature then is increased to 103° C. After thetemperature has become steady at 103° C., HFP is added slowly to thereactor until the pressure is 470 psig (3.34 MPa). 112 ml of liquid PEVEis injected into the reactor. Then TFE is added to the reactor toachieve a final pressure of 630 psig (4.45 MPa). Then 80 ml of freshlyprepared aqueous initiator solution containing 2.20 wt % of ammoniumpersulfate (APS) is charged into the reactor. Then, this same initiatorsolution is pumped into the reactor at a TFE to initiator solution massratio of twenty-three-to-one for the remainder of the polymerizationafter polymerization has begun as indicated by a 10 psi (69 kPa) drop inreactor pressure, i.e. kickoff. Additional TFE is also added to thereactor beginning at kickoff at a goal rate of 0.06 lb/min (0.03 kg/min)subject to limitation in order to prevent the reactor from exceeding themaximum desired limit of 650 psig (4.58 MPa) until a total of 12.0 lb(5.44 kg) of TFE has been added to the reactor after kickoff.Furthermore, liquid PEVE is added to the reactor beginning at kickoff ata rate of 0.2 ml/min for the duration of the reaction.

After 4.0 lb (1.8 kg) of TFE has been fed since kickoff, an aqueoussurfactant solution containing 45,182 ppm of SDS hydrocarbon stabilizingsurfactant and 60,755 ppm of 30% ammonium hydroxide solution is pumpedto the autoclave at a rate of 0.2 ml/min. The aqueous surfactantsolution pumping rate is increased to 0.3 ml/min after 8.0 lb (3.6 kg)of TFE has been fed since kickoff, and finally to 0.4 ml/min after 11.0lb (5.0 kg) of TFE has been fed since kickoff resulting in a total of 28ml of surfactant solution added during reaction. During reaction, thepressure in the reactor reaches the maximum desired limit of 650 psig(4.58 MPa) and the TFE feed rate is reduced from the goal rate tocontrol the pressure. The total reaction time is 266 minutes afterinitiation of polymerization during which 12.0 lb (5.44 kg) of TFE and52 ml of PEVE are added. At the end of the reaction period, the TFEfeed, PEVE feed, the initiator feed and surfactant solution feed arestopped; an additional 100 ml of surfactant solution is added to thereactor, and the reactor is cooled while maintaining agitation. When thetemperature of the reactor contents reaches 90° C., the reactor isslowly vented. After venting to nearly atmospheric pressure, the reactoris purged with nitrogen to remove residual monomer. Upon furthercooling, the dispersion is discharged from the reactor at below 70° C.

Solids content of the dispersion is 20.30 wt % and Dv(50) raw dispersionparticle size (RDPS) is 146.8 nm. 542 grams of wet coagulum is recoveredon cleaning the autoclave. The TFE/HFP/PEVE terpolymer (FEP) has a meltflow rate (MFR) of 16.4 gm/10 min, an HFP content of 11.11 wt %, and aPEVE content of 1.27 wt %, and a melting point of 247.5° C.

FEP-2: Preparation of Hydrocarbon Stabilized TFE/HFP/PEVE Dispersion

A cylindrical, horizontal, water-jacketed, paddle-stirred, stainlesssteel reactor having a length to diameter ratio of about 1.5 and a watercapacity of 10 gallons (37.9 L) is charged with 60 pounds (27.2 kg) ofdeionized water. The reactor temperature then is increased to 103° C.while agitating at 46 rpm. The agitator speed is reduced to 20 rpm andthe reactor is vented for 60 seconds. The reactor pressure is increasedto 15 psig (205 kPa) with nitrogen. The agitator speed is increased to46 rpm while cooling to 80° C. The agitator speed is reduced to 20 rpmand a vacuum is pulled to 12.7 psi (87.6 kPa). A solution containing 500ml of deaerated deionized water, 0.5 grams of Pluronic® 31R1 solutionand 0.3 gm of sodium sulfite is drawn into the reactor. With the reactorpaddle agitated at 20 rpm, the reactor is heated to 80° C., evacuatedand purged three times with TFE. The agitator speed is increased to 46rpm and the reactor temperature then is increased to 103° C. After thetemperature has become steady at 103° C., HFP is added slowly to thereactor until the pressure is 430 psig (3.07 MPa). 112 ml of liquid PEVEis injected into the reactor. Then TFE is added to the reactor toachieve a final pressure of 630 psig (4.45 MPa). Then 80 ml of freshlyprepared aqueous initiator solution containing 2.20 wt % of ammoniumpersulfate (APS) is charged into the reactor. Then, this same initiatorsolution is pumped into the reactor at a TFE to initiator solution massratio of twenty-to-one for the remainder of the polymerization afterpolymerization has begun as indicated by a 10 psi (69 kPa) drop inreactor pressure, i.e. kickoff. Additional TFE is also added to thereactor beginning at kickoff at a rate of 0.06 lb/min (0.03 kg/min)subject to limitation in order to prevent the reactor from exceeding themaximum desired limit of 650 psig (4.58 MPa) until a total of 12.0 lb(5.44 kg) of TFE has been added to the reactor after kickoff.Furthermore, liquid PEVE is added to the reactor beginning at kickoff ata rate of 0.3 ml/min for the duration of the reaction.

After 4.0 lb (1.8 kg) of TFE has been fed since kickoff, an aqueoussurfactant solution containing 45,176 ppm of SDS hydrocarbon stabilizingsurfactant and 60,834 ppm of 30% ammonium hydroxide solution is pumpedto the autoclave at a rate of 0.2 ml/min. The aqueous surfactantsolution pumping rate is increased to 0.3 ml/min after 6.0 lb (2.7 kg)of TFE has been fed since kickoff, then to 0.4 ml/min after 8.0 lb (3.6kg) of TFE has been fed since kickoff, to 0.6 ml/min after 10.0 lb (4.5kg) of TFE has been fed since kickoff, and finally to 0.8 ml/min after11.0 lb (5.0 kg) of TFE has been fed since kickoff resulting in a totalof 47 ml of surfactant solution added during reaction. The totalreaction time is 201 minutes after initiation of polymerization duringwhich 12.0 lb (5.44 kg) of TFE and 60 ml of PEVE are added. At the endof the reaction period, the TFE feed, PEVE feed, the initiator feed andsurfactant solution feed are stopped; an additional 25 ml of surfactantsolution is added to the reactor, and the reactor is cooled whilemaintaining agitation. When the temperature of the reactor contentsreaches 90° C., the reactor is slowly vented. After venting to nearlyatmospheric pressure, the reactor is purged with nitrogen to removeresidual monomer. Upon further cooling, the dispersion is dischargedfrom the reactor at below 70° C.

Solids content of the dispersion is 20.07 wt % and Dv(50) raw dispersionparticle size (RDPS) is 143.2 nm. 703 grams of wet coagulum is recoveredon cleaning the autoclave. The TFE/HFP/PEVE terpolymer (FEP) has a meltflow rate (MFR) of 29.6 gm/10 min, an HFP content of 9.83 wt %, a PEVEcontent of 1.18 wt %, and a melting point of 256.1° C.

Thermally Induced Discoloration

Dried polymer is characterized as described in the Test Methods sectionabove as Measurement of Thermally Induced Discoloration as applicable tothe type of polymer used in the following Examples.

Example 1 Pretreatment of Fluoropolymer Resin by Exposure to OxygenFollowed by Exposure to Fluorine

Aqueous FEP-1 dispersion polymerized as above is coagulated in a heatedglass reactor. 1250 ml of dispersion is heated to 85° C. in a water bathand then transferred to a 2,000 ml jacketed glass reactor with fourinternal baffles produced by Lab Glass or Vineland, N.J. where thetemperature is maintained at by circulating 85° C. water through thejacket. Two high-shear impellers are turned at 2,470 rpm for 3600seconds to cause the dispersion to separate into a polymer phase and awater phase. The water is separated from the solids by filtering througha 150 micron mesh filter bag model NMO150P1SHS manufactured by TheStrainrite Companies of Auburn, Me. The polymer phase is dried for 40hours in a circulating air oven set at 150° C. to produce a dry powder.

A sample of dried powder is molded to produce color films as describedin the Test Methods section above as Measurement of Thermally InducedDiscoloration for melt-processible fluoropolymers to establish the basevalue of L* (L*_(i)=30.5) for untreated color which value is more than49 L units below the L* value of FEP fluoropolymer resin of commercialquality manufactured using ammonium perfluorooctanoate fluorosurfactant,where the standard being used for this example is 79.7. The measuredcolor is shown as “Starting Powder” in Table 1.

All of the experiments are carried out with a 25 mm twin-screw extruder,equipped with an injection probe, which is a rod having a longitudinalbore opening flush with the surface of the extruder barrel in thereaction zone, and a vacuum port connected to a fluorine/hydrofluoricacid scrubbing system. The twin-screw extruder feeds into a 3.81 cm (1.5inch) single-screw extruder, which is equipped with a die. Thetwin-screw extruder serves as a resin melter and end group reactor inwhich the desired end group, and in the case of FEP, backbone,stabilization is conducted. The single-screw extruder serves as a meltpump to generate the pressure necessary to move the resin through theoptional screen pack and die.

The extrusion equipment described above is a “Kombiplast” extruder fromthe Coperion Corporation. Corrosion-resistant materials are used forthose parts that come into contact with the polymer melt andfluorinating agent. The twin-screw extruder has two corotating screwsdisposed side by side. The screw configurations are designed with anintermeshing profile and tight clearances, causing them to beself-wiping. The screw configurations include kneading blocks, mixingelements, and conveying screw bushings. The first 19.4 Length/Diameter(L/D, D being the diameter of the bushings) of the extruder is themelting zone. This contains the feeding, solids conveying, and kneadingblock sections. The kneading block sections provide high shear andinsure proper melting of the polymer. The melting section ends with aleft handed bushing (rearward pumping) that forms a melt seal andinsures complete filling of the final kneading blocks. The reagent isinjected immediately after this section. The next 20.7 L/D contain theinjection, mixing and reaction sections with multiple mixing elementsand constitute the reaction zone of the extruder. The mixing elementsused and their arrangement consist of four working sections with TMEelements followed by a working section with a single ZME element. Thenext 5.4 L/D contains the vacuum extraction section (devolatilizationzone), which is connected to a scrubbing system designed to neutralizeF₂, HF, and other reaction products, depending on the reaction beingcarried out. The vacuum extraction section follows a conventionaldesign, which includes melt forwarding elements that provide for freevolume, so that the molten polymer is exposed to subatmosphericpressure, which prevent reactive and corrosive gases from escaping intothe atmosphere. The vacuum is operated between 55-90 kPa absolute (8 and13 psia). Undercut bushings (SK) are an effective way to provide theforwarding elements in the vacuum extraction section of the extruder.The final 3.3 L/D are used to provide a vacuum seal and pump the moltenpolymer into the single-screw extruder. Chemical reactions mainly occurin the section between the injection nozzle and the vacuum port thatcontains the mixing sections. Backbone stabilization in the case of FEPoccurs in both the kneading block sections and the mixing sections. Thetwin-screw extruder empties into a single-screw melt pump, which isdesigned to generate pressure at low shear rates for filtration andpellet formation. The molten polymer passes through a 0.95 cm (⅜ inch)die hole. The melt strand is then quenched in a water bath to produce asolid strand. The strand is then chopped to produce pellets.

The twin-screw extruder is operated with barrel temperatures of 350° C.and a screw speed of 200 rpm. The single-screw extruder is operated withbarrel temperatures of 350° C. and a screw speed of 20 rpm. The polymeris fed to the extruder at 18 kg/hr.

Dry, compressed air is injected through a nozzle into the injection zoneat an oxygen-to-polymer ratio of 0.10% by weight. The pellets are driedfor 40 hours in a circulating air oven set at 150° C. to remove anyresidual moisture.

The pellets produced by reaction with oxygen from the air injection areprocessed through the extruder again under the same conditions exceptthe air is replaced with a gas that is 10 volume percent fluorine and 90volume percent nitrogen. The gas is injected at a fluorine-to-polymerratio of 0.08% by weight.

The pellets produced with air injection and the pellets produced withair injection followed by fluorine injection are molded to produce colorfilms as described in the Test Methods section above as Measurement ofThermally Induced Discoloration Measurements for melt-processiblefluoropolymers are shown in Table 1. L* obtained after pretreatment withair injection (L*_(t)) is 71.2 with a % change in L* of 82.7% indicatingimproved color over the starting powder. L* obtained after subsequentexposure to fluorine (L*_(t)) is 79.5 with a % change in L* of 99.6%indicating an even greater improvement when both pretreatment andfluorination are combined.

TABLE 1 State L* % change in L* Starting powder 30.5 Pellets producedwith air 71.2 82.7% injection Pellets produced with air 79.5 99.6%injection followed by fluorine injection

Example 2 Pretreatment of Fluoropolymer Dispersion Plus Pretreatment ofFluoropolymer Resin, Subsequent Exposure of Fluoropolymer Resin toFluorine

Aqueous FEP-2 dispersion polymerized as described above is diluted to 5weight percent solids with deionized water. The dispersion is coagulatedby freezing the dispersion at −30° C. for 16 hours. The dispersion isthawed and the water is separated from the solids by filtering through a150 micron mesh filter bag model NMO150P1SHS manufactured by TheStrainrite Companies of Auburn, Me. The solids are dried for 16 hours ina circulating air oven set at 150° C. to produce a dry powder.

A sample of dried powder is molded to produce color films as describedin the Test Methods section above as Measurement of Thermally InducedDiscoloration for melt-processible fluoropolymers to establish the basevalue of L* (L*_(i)=25.9) for untreated color which value is more than53 L units below the L* value of FEP fluoropolymer resin of commercialquality manufactured using ammonium perfluorooctanoate fluorosurfactant,where the standard being used for this example is 79.7. The measuredcolor is shown as “Starting Powder” in Table 2.

Dispersion Pretreatment: 1200 ml of the 5 weight percent solids FEPdispersion described above is preheated to 50° C. in a water bath. Thedispersion and 2 ml of 30 wt % H₂O₂ are added to a 2000 ml jacketedglass reactor with internal diameter of 13.3 cm (5¼ inches), which has50° C. water circulating through the reactor jacket. An impeller withfour 3.18 cm (1.25 inch) long flat blades set at a 45° angle and twoinjection tubes that each have a 12 mm diameter by 24 mm longfine-bubble, fritted-glass cylinder produced by LabGlass as part number8680-130 are placed in the reactor. The injection tubes are connected toan air supply that is passed through a Drierite gas purification columnmodel 27068 produced by W.A. Hammond Drierite Company of Xenia, Ohio andthe air supply is adjusted to deliver 1.42 standard L/min (3.0 standardft³/hr). The agitator is set at 60 rpm. After 5 minutes of mixing, thedispersion temperature is 48.5° C., and the reaction timer is started.After seven hours of reaction, 42 ml of deionized water and 2 ml of 30wt % H₂O₂ are added to replace evaporative losses. The reaction is endedafter 16 hours by stopping the agitator, ceasing the air flow,discontinuing the hot water circulation, and then removing thedispersion from the reactor. The dispersion is coagulated, filtered,dried and molded as described above. The measured color is shown as“Powder after H₂O₂ treatment” in Table 2.

Resin Pretreatment: The solids are dried for 2 hours with 180° C. ozoneenriched air in the equipment described under “Apparatus for Drying ofFEP Polymer” with the use of three AQUA-6 portable ozone generatormanufactured by A2Z Ozone of Louisville, Ky. to discharge ozone throughthree evenly spaced nozzles above the polymer bed. The drying of thefluorpolymer resin with ozone is yet another pretreatment of the resinprior to expose the fluoropolymer to fluorine. The dried powder ismolded to produce color films and measured as described above in theTest Methods above as Measurement of Thermally Induced Discoloration formelt-processible fluoropolymers. The measured color is shown as “Powderafter ozone drying” in Table 2. The drying is repeated to produce 10 kgof dried powder.

The dried powder is pelletized by extruding it through a 28 mmtwin-screw extruder that feeds into a 3.81 cm (1.5 inch) single-screwextruder, which is equipped with a die. The twin-screw extruder servesas a resin melter, and in the case of FEP, backbone stabilization isconducted. The single-screw extruder serves as a melt pump to generatethe pressure necessary to move the resin through the optional screenpack and die. The extrusion equipment described above is a “Kombiplast”extruder from the Coperion Corporation. Corrosion-resistant materialsare used for those parts that come into contact with the polymer melt.The twin-screw extruder has two co-rotating screws disposed side byside. The screw configurations are designed with an intermeshing profileand tight clearances, causing them to be self-wiping. The screwconfigurations include kneading blocks, and conveying screw bushings.The twin-screw extruder empties into a single-screw melt pump, which isdesigned to generate pressure at low shear rates for filtration andpellet formation. The molten polymer passes through a 0.95 cm (⅜ inch)die hole. The melt strand is then quenched in a water bath to produce asolid strand. The strand is then chopped to produce pellets.

The extruders are operated with the barrel temperatures set at 350° C.and screw speeds of 200 rpm for the twin-screw extruder and 20 rpm forthe single-screw extruder. The polymer powder is fed at 9.07 kg/hr (20lb/hr).

Fluorine Exposure: A fluorination reactor is used to further treat thepellets. The fluorination reactor is a modified double-cone blenderequipped with gas inlet and vent connections and an electric heatingmantle as described in U.S. Pat. No. 4,626,587. The reactor is operatedin stationary mode. The fluorination is conducted at 190° C. with 30minutes of operation at a fluorine/nitrogen ratio of 4/96 volumepercent, 30 minutes of operation at a fluorine/nitrogen ratio of 7/93volume percent, and then 360 minutes of operation at a fluorine/nitrogenratio of 10/90 volume percent. At the end of the cycle, fluorine flow isstopped, the electric mantle is turned off, and the reactor isevacuated. The residual fluorine is purged from the reactor withnitrogen.

The powder before pretreatment, powder after H₂O₂ (dispersionpretreatment), powder after ozone drying (resin pretreatment), extrudedpellets, and fluorinated pellets are molded to produce color films asdescribed in the Test Methods section above as Measurement of ThermallyInduced Discoloration for melt-processible fluoropolymers. The measuredcolors are shown in Table 2. L* obtained after pretreatment ofdispersion and isolation of the fluoropolymer resin is 37.4 with a %change in L* of 21.4% indicating much improved color after dispersionpretreatment with H₂O₂. L* obtained after subsequent drying with ozoneis 67.6 with a % change in L* of 77.5% indicating a very much improvedcolor when this second pretreatment is used. L* obtained aftersubsequent exposure to fluorine is 75.9 with a % change in L* of 92.9%indicating an even greater improvement when pretreatment (s) andfluorination are combined. It is also to be noted that the conditions inthe extruder are more aggressive with higher temperature, higher shearrate, and longer residence time than the conditions in the moldingoperation to produce film test chips. The more aggressive conditions inthe extruder result in test chips of extruded pellets which exhibit aninitial decrease in L* as compared to the molded powder sample, prior tothe exposure the polymer resin to fluorine.

TABLE 2 % change in L* Relative to Starting State L* Material StartingPowder 25.9 — Powder after H₂O₂ treatment 37.4 21.4% (DispersionPretreatment) Powder after ozone drying 67.6 77.5% (Resin Pretreatment)Extruded Pellets 61.9 66.9% Fluorinated Pellets 75.9 92.9%

What is claimed is:
 1. Process for reducing thermally induceddiscoloration of fluoropolymer resin, said fluoropolymer resin producedby polymerizing fluoromonomer in an aqueous dispersion medium to formaqueous fluoropolymer dispersion and isolating said fluoropolymer fromsaid aqueous medium by separating fluoropolymer resin in wet form fromthe aqueous medium and drying to produce fluoropolymer resin in dryform, said process comprising: pretreating the aqueous fluoropolymerdispersion and/or the fluoropolymer resin in wet or dry form; andexposing the fluoropolymer resin in dry form to fluorine.
 2. The processof claim 1 wherein said process reduces thermally induced discolorationby at least about 10% as measured by % change in L* on the CIELAB colorscale.
 3. The process of claim 1 wherein said aqueous fluoropolymerdispersion contains hydrocarbon surfactant which causes said thermallyinduced discoloration.
 4. The process of claim 3 wherein saidfluoropolymer dispersion is polymerized in the presence of hydrocarbonsurfactant.
 5. The process of claim 1 wherein said fluoropolymer resinis melt-processible.
 6. The process of claim 5 wherein said exposing ofsaid fluoropolymer resin to fluorine is carried out above the meltingpoint of the fluoropolymer resin.
 7. The process of claim 6 wherein saidexposing the fluoropolymer resin to fluorine is carried out with thefluoropolymer resin heated to a temperature above its melting point upto about 400° C.
 8. The process of claim 1 wherein said exposing of saidfluoropolymer resin to fluorine is carried out below the melting pointof the fluoropolymer resin.
 9. The process of claim 8 wherein saidexposing fluoropolymer resin to fluorine is carried out with thefluoropolymer resin heated to a temperature of about 20° C. to about250° C.
 10. The process of claim 1 wherein the fluoropolymer resin hasan initial thermally induced discoloration value (L*_(i)) at least about4 L units on the CIELAB color scale below the L* value of equivalentfluoropolymer resin of commercial quality manufactured using ammoniumperfluorooctanoate fluorosurfactant.
 11. The process of claim 1 whereinsaid pretreating the aqueous fluoropolymer dispersion and/or thefluoropolymer resin comprises exposing the aqueous fluoropolymerdispersion and/or the fluoropolymer resin to oxidizing agent.
 12. Theprocess of claim 11 wherein said oxidizing agent comprises an oxygensource.
 13. The process of claim 1 wherein the reduction of thermallyinduced discoloration measured by % change in L* on the CIELAB colorscale provided by said pretreating in combination with exposing thefluoropolymer resin in dry form to fluorine is at least about 10%greater than the % change in L* on the CIELAB color scale provided byonly exposing the fluoropolymer resin in dry form to same amount offluorine.
 14. The process of claim 1 wherein, for a given level ofreduction of thermally induced discoloration, said pretreating incombination with exposing the fluoropolymer resin in dry form tofluorine reduces the required quantity of fluorine by at least about 10%compared to only exposing the fluoropolymer resin in dry form tofluorine.